Several documentation updates for the pending 1.2 release:

 * More mentions of new --only-indexes feature
 * Note we now support up to Postgres 9.3, and get rid of outdated list
   of supported operating systems. (As far as we know, pg_repack should
   build on any platforms supported by PostgreSQL itself, although no one
   has tested the Windows build in a long time.)
 * Remove most of the warnings about data corruption possible with concurrent
   DDL: this should no longer be a concern now that we hold an ACCESS SHARE
   lock during full-table repacks. Keep a short warning about old versions
   being susceptible to this problem, just to make clear that it's fixed now
   and as an enticement to upgrade.
 * A few grammar, phrasing, and typo fixes