[DOCS] Reformat uppercase token filter docs (#50555)

* Updates the description and adds a Lucene link
* Adds analyze and custom analyzer snippets

docs/reference/analysis/tokenfilters/uppercase-tokenfilter.asciidoc

@@ -4,5 +4,102 @@
 <titleabbrev>Uppercase</titleabbrev>
 ++++
 
-A token filter of type `uppercase` that normalizes token text to upper
-case.
+Changes token text to uppercase. For example, you can use the `uppercase` filter
+to change `the Lazy DoG` to `THE LAZY DOG`.
+
+This filter uses Lucene's
+https://lucene.apache.org/core/{lucene_version_path}/analyzers-common/org/apache/lucene/analysis/core/UpperCaseFilter.html[UpperCaseFilter].
+
+[WARNING]
+====
+Depending on the language, an uppercase character can map to multiple
+lowercase characters. Using the `uppercase` filter could result in the loss of
+lowercase character information.
+
+To avoid this loss but still have a consistent lettercase, use the <<analysis-lowercase-tokenfilter,`lowercase`>> filter instead.
+====
+
+[[analysis-uppercase-tokenfilter-analyze-ex]]
+==== Example
+
+The following <<indices-analyze,analyze API>> request uses the default
+`uppercase` filter to change the `the Quick FoX JUMPs` to uppercase:
+
+[source,console]
+--------------------------------------------------
+GET _analyze
+{
+  "tokenizer" : "standard",
+  "filter" : ["uppercase"],
+  "text" : "the Quick FoX JUMPs"
+}
+--------------------------------------------------
+
+The filter produces the following tokens:
+
+[source,text]
+--------------------------------------------------
+[ THE, QUICK, FOX, JUMPS ]
+--------------------------------------------------
+
+/////////////////////
+[source,console-result]
+--------------------------------------------------
+{
+  "tokens" : [
+    {
+      "token" : "THE",
+      "start_offset" : 0,
+      "end_offset" : 3,
+      "type" : "<ALPHANUM>",
+      "position" : 0
+    },
+    {
+      "token" : "QUICK",
+      "start_offset" : 4,
+      "end_offset" : 9,
+      "type" : "<ALPHANUM>",
+      "position" : 1
+    },
+    {
+      "token" : "FOX",
+      "start_offset" : 10,
+      "end_offset" : 13,
+      "type" : "<ALPHANUM>",
+      "position" : 2
+    },
+    {
+      "token" : "JUMPS",
+      "start_offset" : 14,
+      "end_offset" : 19,
+      "type" : "<ALPHANUM>",
+      "position" : 3
+    }
+  ]
+}
+--------------------------------------------------
+/////////////////////
+
+[[analysis-uppercase-tokenfilter-analyzer-ex]]
+==== Add to an analyzer
+
+The following <<indices-create-index,create index API>> request uses the
+`uppercase` filter to configure a new 
+<<analysis-custom-analyzer,custom analyzer>>.
+
+[source,console]
+--------------------------------------------------
+PUT uppercase_example
+{
+    "settings" : {
+        "analysis" : {
+            "analyzer" : {
+                "whitespace_uppercase" : {
+                    "tokenizer" : "whitespace",
+                    "filter" : ["uppercase"]
+                }
+            }
+        }
+    }
+}
+--------------------------------------------------