Add info about multiple description templates to default template fields

This an actionable insight from the Code Review FY21-Q2 Category Maturity Scorecard research (insight).

What

Currently the only way for someone to know that it’s possible to add multiple description templates, other than the default, is to read the documentation. There’s a small (?) icon next to the default description templates fields that links to that documentation. There’s also a link in the New MR and New issue pages, next to the Title field, if the project does not have description templates.

Why

Multiple description templates for issues and merge requests is an important feature, especially for large and complex projects. It’s discoverability is quite low, and we have a good opportunity to surface that from the default description template setting.

How

Add clear, visible info about multiple description templates to the default description template fields (for issues and merge requests).