Documentation fix-up: "its" vs. "it's"

Removed ungrammatical apostrophes from documentation, replacing
"it's" with "its" where appropriate (but in a few cases with "it is"
when that read better).

While doing that, I also fixed a couple of minor typos etc.
as I noticed them.

AttributeErrors or KeyErrors that should raise during mapper
configuration due to user errors.  The catch for attribute/keyerror
has been made more specific to not include the configuration step.
fixes #3047

Found using: https://github.com/intgr/topy

a join condition that is strictly from a single column to itself,
translated through some kind of SQL function or expression.  This
is kind of experimental, but the first proof of concept is a
"materialized path" join condition where a path string is compared
to itself using "like".   The :meth:`.Operators.like` operator has
also been added to the list of valid operators to use in a primaryjoin
condition.  fixes #3029

be used to manufacture objects that behave as though they were loaded
from a session, then detached.   Attributes that aren't present
are marked as expired, and the object can be added to a Session
where it will act like a persistent one. fix #3017

- alter this in the unit tests as well as these queries were just copied from the tests
- remove the included_parts.join(parts) from the core CTE doc (also just copied from the
test, where we want to make sure joins don't get screwed up with the CTE) as it doesn't
contribute to the query itself
fixes #3014

the new rules for "where" and "having" woudn't take effect for the
"whereclause" and "having" kw arguments of the :func:`.select` construct,
which is also what :class:`.Query` uses so wasn't working in the
ORM either. fixes #3013 re: #2804