[Expression Builder] Allow the search widget to search in the function description and not only function list

[qgib]

Author Name: Harrissou Santanna (@DelazJ)
Original Redmine Issue: 18033

Redmine category:expressions

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In the Expression builder dialog, the search widget helps you quickly find the function (and its syntax) you may be looking for.
Sometimes, you don't know which function to use nor whether there's one that deal with what you need. It could be nice that the search tool also checks the help panel and shows result from there.

[gacarrillor]

@DelazJ, this was already attempted in #46672, but function tags were preferred over full text search on function descriptions to enhance function discoverability.

If you agree with that, perhaps we could close this issue?

[DelazJ]

Sure