docs: Specified comment in SAR about symbol encoding

[fg-mindee]

This PR specifies the reason of the one hot symbol encoding in the SAR decoder following up on #578.

cc @khalidMindee

[codecov]

Codecov Report

Merging #617 (0b187e2) into main (f97e92b) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #617   +/-   ##
=======================================
  Coverage   96.06%   96.06%           
=======================================
  Files         110      110           
  Lines        4265     4265           
=======================================
  Hits         4097     4097           
  Misses        168      168           

Flag	Coverage Δ
unittests	96.06% <ø> (ø)

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Impacted Files	Coverage Δ
doctr/models/recognition/sar/pytorch.py	98.34% <ø> (ø)
doctr/models/recognition/sar/tensorflow.py	98.49% <ø> (ø)

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[charlesmindee]

thanks!