incomplete DocBlock for get_bookmarks()

[pbiron]

The DocBlock (and hence, the code reference) for ​get_bookmarks() is incomplete.

In particular,

1. the $search param is not mentioned at all
2. the description of the $orderby param leaves out rand and length as possible values, as well as the "shorthands" for the various fields in the wp_links table
3. the description of the $limit param may be misleading, implying that '1+' is a legal value

[SergeyBiryukov]

In 46152:

Docs: Improve documentation for get_bookmarks().

Props pbiron.
Fixes #48053.