sql: clarify the existence of function namespaces and expose the resolution rules

[knz]

This issue is a combined docs+engineering request:

• add an explanation on function and table name resolution in SQL docs - Tell a bit more about SQL name resolution. docs#1050
• add doc pages that explain the special namespaces: crdb_internal, pg_catalog etc
• add a feature to configure the namespace search (SET) and document it
• move pg-specific functions (e.g. unnest, format_type) to ns pg_catalog where they belong
• move standard SQL functions to ns sql where they belong -- then extend the standard search path to include sql
• upgrade the function doc page generator to produce function tables per namespace
• produce a common function list that includes the "promoted" functions -- all those from namespace sql plus some from pg_catalog.
  • add a mechanism to annotate pg_catalog functions as being "promoted" and use that in the generator.

cc @jseldess

[knz]

This has been more or less thoroughly addressed already.