Small doc clearifications on embedding and the GC, plus information on threading restrictions

[paulmelis]

While figuring out some embedding issues in a multi-threaded environment I found the docs to be a bit thin on details. So based on several discourse posts I added a section on thread-safety. Needs a check on correctness/completeness, though.

[vtjnash]

LGTM, but you have a lot of trailing whitespace. Can you fix those?

    git rebase --whitespace=fix master

[inkydragon]

whitespace — Passed

@ViralBShah LGTM

[staticfloat]

I'm not sure how you checked whitespace, but all jobs are now failing with the following error:

Whitespace check found 1 issues:
doc/src/manual/embedding.md:682 -- trailing blank lines
make: *** [Makefile:104: check-whitespace] Error 1

You should use the buildkite whitespace job as the whitespace checker.

[ViralBShah]

Sorry - I merged based on the comment. But the PR status shows that builkite reports whitespace passed.

[paulmelis]

Whitespace check found 1 issues:
doc/src/manual/embedding.md:682 -- trailing blank lines
make: *** [Makefile:104: check-whitespace] Error 1

My patch only touches up to and including line 681, weird?

[vtjnash]

We made the test more strict since CI last ran