Continue imporving the manual.

Document in sql.txt what's not available in sql.
Document readahead better.
Document box.selet_limit().
Cross-reference the data model and iterator description.
Document in SAVE SNAPSHOT that a snapshot can be taken
on SIGUSR1, document what WALs can be deleted after a
snapshot is taken.
Improve some wording in replication.
Add server-administration chapter (a stub yet).
Make .tar.gz install described in the tutorial up to
date with the actual .tar.gz install as of today.
Merge branch 'stable'