Expose stream parameter in public strings find APIs

[davidwendt]

Description

Add stream parameter to public APIs:

• cudf::strings::find()
• cudf::strings::rfind()
• cudf::strings::contains()
• cudf::strings::starts_with()
• cudf::strings::ends_with()
• cudf::strings::findall()
• cudf::strings::find_multiple()

Also cleaned up some of the doxygen comments.

Reference #13744

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[vyasr]

Could we add tests for the new stream-based APIs (see any of my recent PRs for what those should look like)? They're mostly just to be safe, but on occasion they have revealed places where we're internally not forwarding input streams along and are just passing cudf::get_default_stream() instead.

[davidwendt]

/merge