constructs are now importable from the "from sqlalchemy" namespace,
just like every other Core construct.
- The implicit conversion of strings to :func:`.text` constructs
when passed to most builder methods of :func:`.select` as
well as :class:`.Query` now emits a warning with just the
plain string sent.   The textual conversion still proceeds normally,
however.  The only method that accepts a string without a warning
are the "label reference" methods like order_by(), group_by();
these functions will now at compile time attempt to resolve a single
string argument to a column or label expression present in the
selectable; if none is located, the expression still renders, but
you get the warning again. The rationale here is that the implicit
conversion from string to text is more unexpected than not these days,
and it is better that the user send more direction to the Core / ORM
when passing a raw string as to what direction should be taken.
Core/ORM tutorials have been updated to go more in depth as to how text
is handled.
fixes #2992

for ticket 2992.

the ORM one in particular was really long winded and I don't really care if people
use text() anyway, they'll figure it out ;)

rather than straight strings.  reference #2992

- add language to MATCH explicitly stating this operator varies by backend and is not available
on SQLite, as the tutorial defaults to SQLite to start with, fix #3059
- on the actual match() documentation fix this up to be more accurate, list some example renderings
for different backends.  again mention SQLite not supported

to accept things flexibly), fix #2999