[1.6.x] Fixed #21427 -- Clearly state integer field value ranges in docs

Added an explicit mention of the exact value ranges for integer type
fields that are safe in all databases supported by Django. Also, put
all value numbers inside double ticks.

Backport of b6a6cf4ab791b092e18ab16537aa88dd7ae9bc36 from master.
[1.6.x] Fixed #21431 -- GenRel->FK list_filter regression in admin

Report, analysis and tests from stephenmcd.

Backpatch of 752d3d70da6291039f33781a0a2ef6f3b7c5fcb5 from master.
[1.6.x] Fixed #21405 -- Prevented queryset overwrite in BaseModelAdmin

Thanks guido@20tab.com for the report and Tim Graham for the
analyze.
Backport of 1718b5256c from master.