pgsql: doc: Simplify description of functions for pg_walinspect

doc: Simplify description of functions for pg_walinspect

As introduced in 2258e76, the docs were hard to parse:
- The examples used listed a lot of long records, bloating the output.
These are switched to show less records with the expanded format,
similarly to pageinspect.
- The function descriptions listed all the OUT parameters, producing
long lines. This is updated so as only the input parameters are
documented, clarifying the whole.
- Remove one example on pg_get_wal_stats() when per_record is set to
true, which is not really necessary once we know the output produced,
and the behavior of the parameter is documented.

While on it, fix a few grammar mistakes and simplify a couple of
sentences.

Author: Bharath Rupireddy
Discussion: https://postgr.es/m/CALj2ACVGcUpziGgQrcT-1G3dHWQQfWjYBu1YQ2ypv9y86dgogg@mail.gmail.com
Backpatch-through: 15

Branch
------
REL_15_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/93595ffd5964247198699339fd8af19c1e4b6a33

Modified Files
--------------
doc/src/sgml/pgwalinspect.sgml | 181 ++++++++++++-----------------------------
1 file changed, 54 insertions(+), 127 deletions(-)