Rewrite description for URL alias on node/add/*

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Problem/Motivation

The current help text is confusing.
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

Change the help text to

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

Test patches, Write a patch.

User interface changes

The help text on URL settings tab changes.

API changes

none.

Data model changes

none.