General: Remove or add inline comments to `$HTTP_RAW_POST_DATA` occurrences.

The `$HTTP_RAW_POST_DATA` global was deprecated in PHP 5.6 and removed completely in PHP 7.0. In general, `php://input` should be used instead of `$HTTP_RAW_POST_DATA`.

Because WordPress Core still supports PHP 5.6, some plugins or sites may still rely on this variable being present and populated with the expected data. For that reason, occurrences of the variable will remain with updated inline documentation until support for PHP 5.6 is officially dropped in WordPress.

Props skoskie, jrf, desrosj, TimothyBlynJacobs.
See #49922.
Fixes #49810.
Built from https://develop.svn.wordpress.org/trunk@47926


git-svn-id: https://core.svn.wordpress.org/trunk@47700 1a063a9b-81f0-0310-95a4-ce76da25c4cd