Rewrite description for URL alias on node/add/*

Problem/Motivation

The current wording is confusing and too long. The proposed solution offers a shortened and clearer text.

Current:(Figure)
Optionally specify an alternative URL by which this content can be accessed. For example, type "about" when writing an about page. Use a relative path and don't add a trailing slash or the URL alias won't work.

Proposed resolution
The alternative URL for this content. For example, type "about" when creating an about page. Use a relative path without a trailing slash

Remaining tasks
Needs a patch with the text change.

User interface changes
The description text for the URL alias field under URL path settings when adding new content.

API changes
None.

Data model changes
None.