rustdoc: Rename `@has FILE PATTERN` to `@hastext`

[camelid]

@has FILE PATTERN treats PATTERN as literal text, while @has FILE PATTERN ANOTHER_PATTERN treats PATTERN as an XPath (and ANOTHER_PATTERN as literal text). This is quite confusing and left me perplexed at why my test was failing. IMO we should have a different name for these two versions. The literal (arity-2) version could be named @hastext.

This would also rename the equivalent for @matches to @matchestext.

Originally posted by @camelid in #100348 (comment)

[camelid]

Currently @has FILE PATTERN '' acts how I would've expected @has FILE PATTERN to work. I.e., it treats PATTERN as an XPath but allows any text. After this issue is implemented, we could change @has FILE PATTERN to mean exactly that.