Collected documentation updates from review comments

[mborland]

Close: #131
Close: #133
Close: #137

[cppalliance-bot]

An automated preview of the documentation is available at https://145.charconv.prtest.cppalliance.org/libs/charconv/doc/html/index.html

[cppalliance-bot]

An automated preview of the documentation is available at https://145.charconv.prtest.cppalliance.org/libs/charconv/doc/html/index.html

[cppalliance-bot]

An automated preview of the documentation is available at https://145.charconv.prtest.cppalliance.org/libs/charconv/doc/html/index.html

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (df09611) 88.29% compared to head (7a9644e) 88.40%.
Report is 4 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #145      +/-   ##
===========================================
+ Coverage    88.29%   88.40%   +0.11%     
===========================================
  Files           56       56              
  Lines         7919     7935      +16     
===========================================
+ Hits          6992     7015      +23     
+ Misses         927      920       -7     

Files	Coverage Δ
.../boost/charconv/detail/from_chars_integer_impl.hpp	100.00% <100.00%> (ø)
test/from_chars.cpp	100.00% <100.00%> (ø)
test/from_chars_float.cpp	98.59% <100.00%> (+<0.01%)	⬆️

... and 1 file with indirect coverage changes

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