Allow multiple slugs in get_terms

[jfarthing84]

Just as the title says, sometimes I need to retrieve multiple term IDs from slugs. Rather than calling get_term_by over and over, it'd be better to be able to specify an array of slugs for get_terms.

[jfarthing84]

Allow array of slugs in get_terms.

[jfarthing84]

Refresh patch against trunk.

[boonebgorges]

Thanks for the refresh! It would be nice to have unit tests for the new functionality.

[dlh]

23636.2.patch fixes a syntax typo in the previous patch and adds unit tests.

[boonebgorges]

In 30042:

Allow 'slug' param of get_terms() to accept an array.

Props jfarthing84, dlh.
Fixes #23636.

[DrewAPicture]

Unfortunately, the new parameter description for the slug argument is too vague. We need to say "slug or array of slugs" specifically.

[jfarthing84]

Make the 'slug' parameter description more verbose.

[jared_smith]

23636.2.patch​ contains unit tests, so I'm removing the "needs-unit-tests" keyword.

[boonebgorges]

In 30086:

Be more specific in documentation of 'slug' param for get_terms().

Props jfarthing84.
Fixes #23636.