add doc

website/pages/docs/from-1.5/blocks/variable.mdx

@@ -24,6 +24,8 @@ If a default value is set, the variable is optional. Otherwise, the variable
 
 `@include 'from-1.5/variables/assignment.mdx'`
 
+`@include 'from-1.5/variables/custom-validation.mdx'`
+
 Example of a variable assignment from a file:
 
 `@include 'from-1.5/variables/foo-pkrvar.mdx'`

website/pages/docs/from-1.5/variables.mdx

@@ -162,6 +162,8 @@ maintainers, use comments.
 
 The following sections describe these options in more detail.
 
+`@include 'from-1.5/variables/custom-validation.mdx'`
+
 ### Variables on the Command Line
 
 To specify individual variables on the command line, use the `-var` option when

website/pages/partials/from-1.5/variables/custom-validation.mdx

@@ -0,0 +1,43 @@
+## Custom Validation Rules
+
+In addition to Type Constraints, you can specify arbitrary custom validation
+rules for a particular variable using one or more `validation` block nested
+within the corresponding `variable` block:
+
+```hcl
+variable "image_id" {
+  type        = string
+  description = "The id of the machine image (AMI) to use for the server."
+
+  validation {
+    condition     = length(var.image_id) > 4 && substr(var.image_id, 0, 4) == "ami-"
+    error_message = "The image_id value must be a valid AMI id, starting with \"ami-\"."
+  }
+}
+```
+
+The `condition` argument is an expression that must use the value of the
+variable to return `true` if the value is valid or `false` if it is invalid.
+The expression can refer only to the variable that the condition applies to,
+and _must not_ produce errors.
+
+If the failure of an expression is the basis of the validation decision, use
+[the `can` function](./functions/can.html) to detect such errors. For example:
+
+```hcl
+variable "image_id" {
+  type        = string
+  description = "The id of the machine image (AMI) to use for the server."
+
+  validation {
+    # regex(...) fails if it cannot find a match
+    condition     = can(regex("^ami-", var.image_id))
+    error_message = "The image_id value must be a valid AMI id, starting with \"ami-\"."
+  }
+}
+```
+
+If `condition` evaluates to `false`,  an error message including the sentences
+given in `error_message` will be produced. The error message string should be
+at least one full sentence explaining the constraint that failed, using a
+sentence structure similar to the above examples.