pgsql: Correct comment and some documentation about REPLICA_IDENTITY_IN

Correct comment and some documentation about REPLICA_IDENTITY_INDEX

catalog/pg_class.h was stating that REPLICA_IDENTITY_INDEX with a
dropped index is equivalent to REPLICA_IDENTITY_DEFAULT. The code tells
a different story, as it is equivalent to REPLICA_IDENTITY_NOTHING.

The behavior exists since the introduction of replica identities, and
fe7fd4e even added tests for this case but I somewhat forgot to fix this
comment.

While on it, this commit reorganizes the documentation about replica
identities on the ALTER TABLE page, and a note is added about the case
of dropped indexes with REPLICA_IDENTITY_INDEX.

Author: Michael Paquier, Wei Wang
Reviewed-by: Euler Taveira
Discussion: https://postgr.es/m/OS3PR01MB6275464AD0A681A0793F56879E759@OS3PR01MB6275.jpnprd01.prod.outlook.com
Backpatch-through: 10

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/861095a40987fb40d0e3ed43f223ca0f79071dea

Modified Files
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doc/src/sgml/ref/alter_table.sgml | 51 +++++++++++++++++++++++++++++++++------
src/include/catalog/pg_class.h | 2 +-
2 files changed, 44 insertions(+), 9 deletions(-)