Improve documentation about MVCC-unsafe utility commands.

 Improve documentation about MVCC-unsafe utility commands.
 
 The table-rewriting forms of ALTER TABLE are MVCC-unsafe, in much the same
 way as TRUNCATE, because they replace all rows of the table with newly-made
 rows with a new xmin.  (Ideally, concurrent transactions with old snapshots
 would continue to see the old table contents, but the data is not there
 anymore --- and if it were there, it would be inconsistent with the table's
 updated rowtype, so there would be serious implementation problems to fix.)
 This was nowhere documented though, and the problem was only documented for
 TRUNCATE in a note in the TRUNCATE reference page.  Create a new "Caveats"
 section in the MVCC chapter that can be home to this and other limitations
 on serializable consistency.
 
 In passing, fix a mistaken statement that VACUUM and CLUSTER would reclaim
 space occupied by a dropped column.  They don't reconstruct existing tuples
 so they couldn't do that.
 
 Back-patch to all supported branches.