Update API schedule option description - 4.3 plus

Updated line 627 to "For use with CiviCRM 4.3 and later," per info from @totten

Update documentation on documentation

Added Chris Burgess' and Sean Madsen's contributions from StackExchan…

Closes #130: Clarity around "overrides" language.

Update pr-review.md

Add links to other documentation, clean up grammar and formatting, clarify a couple points

Remove invalid (outdated?) requirement that extensions provide documentation.

WIP PR Review

fix broken link to blog post

Update extend-stages.md

filesystem.md - Multiple revisions

I've seen a incidents where an extension developer tries to hardcode paths
(based on their knowledge/understanding of the file-structure).  This is
usually a bad idea, so I'd like to expose the programmatic way as part of
the docs.

When we first ported to Backdrop, Jen+Nate wanted the default file structure
to be a bit different from Drupal (e.g.  `/civicrm.settings.php` instead of
`/sites/default/civicrm.settings.php`; `/modules/civicrm` instead of
`/sites/all/modules/civicrm`).

I think the Backdrop folks have a slightly different approach toward
multisite than the D7 folks: IIRC, the multisite stuff was still there
functionally, but it wasn't the default.

Separate file system doc