Revamp select documentation

[VersusFacit]

--select and --models flags need significantly improved documentation both for dbt Labs folks and community folks.

The particular case that spawned this issue (although it's come up plenty of times in the past):

It's unclear when to use one or or the other and with which commands. We should have a page that summarizes the grammar and functionality of different command/option combinations.

Case and point:

[jtcohen6]

Right on!

There is a table here of which flags go with which command: https://docs.getdbt.com/reference/node-selection/syntax

Ian is also totally right, we want to make this --select across the board (--models will still work, it just shouldn't be documented): dbt-labs/dbt-core#3210

[jtcohen6]

Going to consider this resolved by v0.21 and #802 / 58b8ed2