doc(bigtable): deprecate RowReader public ctors

[dbolduc]

The deprecation part of #8854

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This change is 

[google-cloud-cpp-bot]

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Codecov Report

Merging #8887 (07da1d1) into main (13e5b01) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #8887      +/-   ##
==========================================
- Coverage   93.16%   93.16%   -0.01%     
==========================================
  Files        1477     1477              
  Lines      124810   124836      +26     
==========================================
+ Hits       116283   116302      +19     
- Misses       8527     8534       +7     

Impacted Files	Coverage Δ
google/cloud/bigtable/row_reader.cc	26.31% <ø> (-73.69%)	⬇️
google/cloud/bigtable/version.h	100.00% <ø> (ø)
google/cloud/bigtable/row_reader.h	100.00% <100.00%> (ø)
google/cloud/bigtable/row_reader_test.cc	100.00% <100.00%> (ø)
google/cloud/bigtable/table.cc	99.25% <100.00%> (+0.01%)	⬆️
google/cloud/bigtable/async_read_stream_test.cc	97.99% <0.00%> (+0.66%)	⬆️
google/cloud/pubsub/subscriber_connection_test.cc	97.91% <0.00%> (+0.69%)	⬆️
...cloud/pubsub/internal/subscription_session_test.cc	98.75% <0.00%> (+0.74%)	⬆️

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[coryan]

I cannot improve on your choice of tag (doc:) but it seems weird.

[coryan]

Unrelated to this change, the lack of default constructor is annoying. It makes it impossible to write code like this:

RowReader reader;
if (condition) {
  reader = table.ReadRows(...);
} else {
  reader = table.ReadRows(... /* but slightly different dots */);
}

[dbolduc]

Opened #8892

[dbolduc]

I cannot improve on your choice of tag (doc:) but it seems weird.

That is probably a sign that I should have split this up into two PRs.

• a refactor: that introduces bigtable_internal::MakeRowReader()
• a doc: that adds compiler warnings, CHANGELOG entry, etc.