Fixes for hooks documentation on xmlrpc.php.

Adds spacing, changes an `@see` to `@link`, adds a docs-specific variable for a parameter.

See #26869, #25229 and [25281].

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git-svn-id: https://core.svn.wordpress.org/trunk@27013 1a063a9b-81f0-0310-95a4-ce76da25c4cd