Use the field's help text when set

After discussion with other developers on the project, I opted to not completely replace the help text but to concatenate it, both to have consistency with the help test across all Linkit widgets and to minimally affect existing fields.

This seems like a good compromise. An additional enhancement to communicate this novel behavior to Site Builders might be to add some explanatory text to the Widget settings. Maybe something like:

Help text for URL field
The following default help text will be displayed for the URL field. "Start typing to find content or paste a URL and click on the suggestion below."
If you add your own help text in the field settings, it will display after the default help text.

This could be inserted between the Placeholder for URL and Placeholder for link text.

Let me add another aspect to the discussion:
I wonder why Linkit's widget uses a fixed help text for all cases, in contrast to Core's LinkWidget, which has 3 different help texts, depending on the link type setting (internal links only, external links only, internal and external links allowed).

"Start typing to find content" doesn't really make sense when only external links are allowed; same goes for "paste a URL" with internal links only.

Not sure if this is the right place for that addition, or we should create a separate issue.