Docs for Locking reads

[dereuromark]

We should add docs for propelorm/Propel2#1658 etc

[kasparsatke]

As far as I can see it, the only documentation for the files covered by this PR is on the API docs under http://api.propelorm.org/2.0-master/ respectively https://github.com/propelorm/api.propelorm.org - which have not been updated since almost 11 years.

In propelorm/Propel2#2006 I have just been suggesting to merge/unify the API docs with the main docs. And also to update the API docs of course 😀.

Opinions on this are welcome!