document SQL Server MAX symbol behavior

[Mike Bayer]

per #3759, users don't have any documentation pointing them to how to get SQL server to do "MAX" w/ VARCHAR, as the base types won't handle this. they need to use the SQL Server type and pass in None, this needs docs.

[Mike Bayer]

correction, there is no sql server VARCHAR / NVARCHAR, we use MAX for the "None" length.

[Mike Bayer]

• Document how SQL Server does MAX with VARCHAR, NVARCHAR Fixes #3760

Change-Id: I0613eb66bfdc9d7118688c74e29c8da322c3b4db

→ <<cset 0d92f79fd860>>

[Mike Bayer]

• Document how SQL Server does MAX with VARCHAR, NVARCHAR Fixes #3760

Change-Id: I0613eb66bfdc9d7118688c74e29c8da322c3b4db (cherry picked from commit 0d92f79fd86073203a2a956460140c311c85a396)

→ <<cset 9288d691f3a5>>

[Mike Bayer]

see also #3504 #3759