Ask for a float from microtime() for timer_start(), timer_stop(). Clarify...

Ask for a float from microtime() for timer_start(), timer_stop(). Clarify docs. props solarissmoke, fixes #19157.

git-svn-id: https://core.svn.wordpress.org/trunk@19611 1a063a9b-81f0-0310-95a4-ce76da25c4cd