Update API-functions.md description to be more explicit about grpdata_size updates

[clime]

Small tweak of the docs regarding SRT bonding :).

[codecov-commenter]

Codecov Report

Merging #2820 (d8f709b) into master (33a620b) will decrease coverage by 0.13%.
Report is 19 commits behind head on master.
The diff coverage is 70.43%.

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@@            Coverage Diff             @@
##           master    #2820      +/-   ##
==========================================
- Coverage   67.05%   66.92%   -0.13%     
==========================================
  Files         102      102              
  Lines       20269    20367      +98     
==========================================
+ Hits        13591    13631      +40     
- Misses       6678     6736      +58     

Files	Coverage Δ
srtcore/buffer_rcv.cpp	70.66% <100.00%> (-0.08%)	⬇️
srtcore/buffer_snd.cpp	68.70% <100.00%> (ø)
srtcore/buffer_snd.h	50.00% <ø> (ø)
srtcore/buffer_tools.h	36.36% <ø> (ø)
srtcore/common.cpp	34.92% <100.00%> (ø)
srtcore/common.h	87.85% <100.00%> (+0.14%)	⬆️
srtcore/congctl.cpp	81.44% <100.00%> (+0.61%)	⬆️
srtcore/epoll.cpp	62.82% <100.00%> (ø)
srtcore/fec.cpp	43.25% <100.00%> (ø)
srtcore/group.cpp	37.99% <100.00%> (-0.10%)	⬇️
... and 28 more

... and 2 files with indirect coverage changes

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