Taglib docs: Add the checkMethod attribute to textbox (#3396)

* Add the checkMethod attribute to textbox
- better discovery capability for the plugin maintainers and core developers

* Address Daniel's comment about the possible values

* Address Devin's comment about the documentation content

core/src/main/resources/lib/form/textarea.jelly

@@ -51,8 +51,9 @@ THE SOFTWARE.
       If @field is specified, this will be inferred automatically,
       which is the recommended approach.
     </st:attribute>
-    <st:attribute name="checkMethod">
-      If specified, the HTTP method to use for input field will be checked (via AJAX)
+    <st:attribute name="checkMethod" use="optional" type="String">
+      Specify 'post' (must be lowercase) to change the HTTP method used for the AJAX requests to @checkUrl from a GET to a POST.
+      If any other value is specified then requests will use GET.
     </st:attribute>
     <st:attribute name="codemirror-mode">
       Turns this text area into CodeMirror-assisted code editing text area.

core/src/main/resources/lib/form/textbox.jelly

@@ -64,6 +64,10 @@ THE SOFTWARE.
       If @field is specified, this will be inferred automatically,
       which is the recommended approach.
     </st:attribute>
+    <st:attribute name="checkMethod" use="optional" type="String">
+      Specify 'post' (must be lowercase) to change the HTTP method used for the AJAX requests to @checkUrl from a GET to a POST. 
+      If any other value is specified then requests will use GET.
+    </st:attribute>
     <st:attribute name="autoCompleteDelimChar">
       A single character that can be used as a delimiter for autocompletion. Normal
       autocomplete will replace the entire content of the text box with the autocomplete