[DOCS] Update model_memory_limit (elastic/x-pack-elasticsearch#1928)

* [DOCS] Update model_memory_limit

* [DOCS] Clarify minimum model_memory_limit value

* [DOCS] More updates to model_memory_limit

* [DOCS] Address feedback in jobresource.asciidoc

Original commit: elastic/x-pack-elasticsearch@3c62719037

docs/en/ml/getting-started.asciidoc

@@ -483,12 +483,13 @@ The number of records that have been processed by the job.
 Memory status::
 The status of the mathematical models. When you create jobs by using the APIs or
 by using the advanced options in {kib}, you can specify a `model_memory_limit`.
-That value is the maximum amount of memory resources, in MiB, that the
-mathematical models can use. Once that limit is approached, data pruning becomes
-more aggressive. Upon exceeding that limit, new entities are not modeled.
-The default value is `4096`. The memory status field reflects whether you have
-reached or exceeded the model memory limit. It can have one of the following
-values: +
+That value is the maximum amount of memory resources that the mathematical
+models can use. Once that limit is approached, data pruning becomes more
+aggressive. Upon exceeding that limit, new entities are not modeled. For more
+information about this setting, see
+{ref}/ml-job-resource.html#ml-apilimits[Analysis Limits]. The memory status
+field reflects whether you have reached or exceeded the model memory limit. It
+can have one of the following values: +
 `ok`::: The models stayed below the configured value.
 `soft_limit`::: The models used more than 60% of the configured memory limit
 and older unused models will be pruned to free up space.

docs/en/rest-api/ml/jobresource.asciidoc

@@ -277,26 +277,33 @@ For more information, see
 //<<ml-configuring-categories>>.
 
 `model_memory_limit`::
-  (long) The approximate maximum amount of memory resources that are required
-  for analytical processing, in MiB. Once this limit is approached, data pruning
+  (long or string) The approximate maximum amount of memory resources that are
+  required for analytical processing. Once this limit is approached, data pruning
   becomes more aggressive. Upon exceeding this limit, new entities are not
-  modeled. The default value is 4096.
+  modeled. The default value is `4096mb`. If you specify a number instead of a
+  string, the units are assumed to be MiB. Specifying a string is recommended
+  for clarity. If you specify a byte size unit of `b` or `kb` and the number
+  does not equate to a discrete number of megabytes, it is rounded down to the
+  closest MiB. The minimum valid value is 1 MiB. If you specify a value less
+  than 1 MiB, an error occurs. For more information about supported byte size
+  units, see
+  {ref}/common-options.html#byte-units[Byte size units].
 
 [float]
 [[ml-apimodelplotconfig]]
 ==== Model Plot Config
 
 This advanced configuration option stores model information along with the
-results. It provides a more detailed view into anomaly detection. 
+results. It provides a more detailed view into anomaly detection.
 
-WARNING: If you enable model plot it can add considerable overhead to the performance 
+WARNING: If you enable model plot it can add considerable overhead to the performance
 of the system; it is not feasible for jobs with many entities.
 
 Model plot provides a simplified and indicative view of the model and its bounds.
 It does not display complex features such as multivariate correlations or multimodal data.
 As such, anomalies may occasionally be reported which cannot be seen in the model plot.
 
-Model plot config can be configured when the job is created or updated later. It must be 
+Model plot config can be configured when the job is created or updated later. It must be
 disabled if performance issues are experienced.
 
 The `model_plot_config` object has the following properties:
@@ -306,7 +313,7 @@ The `model_plot_config` object has the following properties:
   each entity that is being analyzed. By default, this is not enabled.
 
 `terms`::
-  (string) Limits data collection to this comma separated list of partition or by field values. 
-  If terms are not specified or it is an empty string, no filtering is applied. 
+  (string) Limits data collection to this comma separated list of partition or by field values.
+  If terms are not specified or it is an empty string, no filtering is applied.
   For example, "CPU,NetworkIn,DiskWrites". This is experimental. Only the specified `terms` can
   be viewed when using the Single Metric Viewer.

docs/en/rest-api/ml/update-job.asciidoc

@@ -23,8 +23,7 @@ The following properties can be updated after the job is created:
 |Name |Description |Requires Restart
 
 |`analysis_limits`: `model_memory_limit` |The approximate maximum amount of
-memory resources required for analytical processing, in MiB.
-See <<ml-apilimits>>. | Yes
+memory resources required for analytical processing. See <<ml-apilimits>>. | Yes
 
 |`background_persist_interval` |Advanced configuration option. The time between
 each periodic persistence of the model. See <<ml-job-resource>>. | Yes