Add docs about path param restrictions (hashicorp#13413)

* Add docs about path param restrictions

* Update website/content/api-docs/auth/userpass.mdx

Co-authored-by: Loann Le <84412881+taoism4504@users.noreply.github.com>

* Update with review suggestion

Co-authored-by: Loann Le <84412881+taoism4504@users.noreply.github.com>

website/content/api-docs/auth/userpass.mdx

@@ -26,7 +26,7 @@ Create a new user or update an existing user. This path honors the distinction b
 
 ### Parameters
 
-- `username` `(string: <required>)` – The username for the user. Accepted characters: alphanumeric plus "_", "-", "." (underscore, hyphen and period); username cannot begin with hyphen or period.
+- `username` `(string: <required>)` – The username for the user. Accepted characters: alphanumeric plus "_", "-", "." (underscore, hyphen and period); username cannot begin with a hyphen, nor can it begin or end with a period.
 - `password` `(string: <required>)` - The password for the user. Only required
   when creating the user.
 

website/content/api-docs/index.mdx

@@ -47,6 +47,11 @@ Responses from auth login methods that generate an authentication token are
 sent back to the client via JSON. The resulting token should be saved on the
 client or passed via the `X-Vault-Token` or `Authorization` header for future requests.
 
+## Parameter restrictions
+
+Several Vault APIs require specifying path parameters. The path parameter cannot end 
+in periods. Otherwise, Vault will return a 404 unsupported path error.
+
 ## Namespaces
 
 If using the [Namespaces](/docs/enterprise/namespaces) feature, API