Doc: clarify n_distinct_inherited setting

Doc: clarify n_distinct_inherited setting

There was some confusion around how to adjust the n_distinct estimates
for partitioned tables.  Here we try and clarify that
n_distinct_inherited needs to be adjusted rather than n_distinct.

Also fix some slightly misleading text which was talking about table
size rather than table rows, fix a grammatical error, and adjust some
text which indicated that ANALYZE was performing calculations based on
the n_distinct settings.  Really it's the query planner that does this
and ANALYZE only stores the overridden n_distinct estimate value in
pg_statistic.

Author: David Rowley <[email protected]>
Reviewed-by: David G. Johnston <[email protected]>
Reviewed-by: Chao Li <[email protected]>
Backpatch-through: 13
Discussion: https://postgr.es/m/CAApHDvrL7a-ZytM1SP8Uk9nEw9bR2CPzVb+uP+bcNj=_q-ZmVw@mail.gmail.com