Make explicit primary key requirement in docs for automap table reflection

[Sam Zhang]

The FAQ/ORM section makes clear that a primary key is required for a table to be mapped: http://docs.sqlalchemy.org/en/latest/faq/ormconfiguration.html#how-do-i-map-a-table-that-has-no-primary-key.

However this was unclear from the automap documentation, as can be seen in the following StackOverflow questions: http://stackoverflow.com/questions/23765681/sqlalchemy-automap-does-not-create-class-for-tables-without-primary-key http://stackoverflow.com/questions/29905160/automap-reflect-tables-within-a-postgres-schema-with-sqlalchemy (me)

and on the mailing list: https://groups.google.com/forum/#!topic/sqlalchemy/8F2tPkpR4bE

[Mike Bayer]

• add boldface for "viable" plus a note describing that this refers to the table having a primary key. fixes #3398

→ <<cset 93b5eae9843d>>

[Mike Bayer]

• add boldface for "viable" plus a note describing that this refers to the table having a primary key. fixes #3398

(cherry picked from commit 93b5eae9843d423378f68be928a4f1e6fcacfb87)

→ <<cset 933af1078f93>>

[Mike Bayer]

added a note. thanks for reporting!