Clarify docs on checkpoints in async iterables

[Zac-HD]

Closes #2388

[codecov[bot]]

Codecov Report

Merging #2390 (9e8544d) into master (2d62ff0) will increase coverage by 0.03%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2390      +/-   ##
==========================================
+ Coverage   98.98%   99.02%   +0.03%     
==========================================
  Files         119      119              
  Lines       16165    16165              
  Branches     3101     3101              
==========================================
+ Hits        16001    16007       +6     
+ Misses        114      111       -3     
+ Partials       50       47       -3     

Impacted Files	Coverage Δ
trio/tests/test_ssl.py	99.86% <0.00%> (+0.55%)	:arrow_up:
trio/_highlevel_ssl_helpers.py	100.00% <0.00%> (+11.76%)	:arrow_up:

[Sxderp]

nit: I think that comma is unnecessary.

[Zac-HD]

Comma might not be necessary, but I like it 🤷‍♂️ Merging because there's no danger of breaking APIs for a docs change, there are no major objections, and I'd like to move on.