Document string schema custom attributes

This commit adds changes to document string schema custom attributes. Community at truecharts raised a concern where they are not sure how the regex works for valid_char string attribute, so the changes introduced in this commit address the concerns and while we are here document other custom string options as well.

docs/schema/string.md

+## String Schema
+
+
+#### Example of string schema options:
+```
+schema:
+    type: string
+    private: true
+    min_length: 5
+    max_length: 12
+    valid_chars: "[a-zA-Z0-9]$"
+```
+
+Following attributes can be added to string schema to enforce validation when a chart release is being created/edited:
+
+`min_length` is an integer field which adds validation to ensure value provided for this variable is at least the specified
+characters.
+
+`max_length` is an integer field which adds validation to ensure value provided for this variable is at max the specified
+characters.
+
+`private` is a boolean field which when set by chart maintainer, will result in the value of the variable not being
+shown to the user in the UI. This is useful for sensitive fields like password where a dummy character is placed for
+each character provided for this string in the UI.
+
+`valid_chars` is a string field which chart maintainer can specify to provide a regex to enforce that the value provided
+for the variable should conform to a pattern. TrueNAS Scale will use python3 regex (https://docs.python.org/3/library/re.html)
+syntax to enforce `valid_chars`.