add a docs cross-reference between adding columns & relationships to existing table...

add a docs cross-reference between adding columns & relationships to existing table mappings

For context see discussion https://github.com/sqlalchemy/sqlalchemy/discussions/11124. This change adds the requested cross-reference to the documentation.

This pull request is:

- [X] A documentation / typographical / small typing error fix
	- Good to go, no issue or tests are needed
- [ ] A short code fix
	- please include the issue number, and create an issue if none exists, which
	  must include a complete example of the issue.  one line code fixes without an
	  issue and demonstration will not be accepted.
	- Please include: `Fixes: #<issue number>` in the commit message
	- please include tests.   one line code fixes without tests will not be accepted.
- [ ] A new feature implementation
	- please include the issue number, and create an issue if none exists, which must
	  include a complete example of how the feature would look.
	- Please include: `Fixes: #<issue number>` in the commit message
	- please include tests.

**Have a nice day!**

Closes: #11133
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/11133
Pull-request-sha: f8cc7a9c510f058f75bcb4308f3f398b8ae70de8

Change-Id: Ic683354fa05560d869d47ceda820d88e758e2973
(cherry picked from commit ba9c86f2075bd4eb1d71caca58b6da6fe19e35ac)