doc: Clean up some recently added PL/pgSQL documentation
doc: Clean up some recently added PL/pgSQL documentation

- Capitalize titles consistently.
- Fix some grammar.
- Group "Obtaining Information About an Error" under "Trapping Errors",
  but make "Obtaining the Call Stack Context Information" its own
  section, since it's not about errors.

techdragon/Postgres-XL,Postgres-XL/Postgres-XL,jmcatamney/gpdb,ashwinstar/gpdb,50wu/gpdb,ashwinstar/gpdb,adam8157/gpdb,ovr/postgres-xl,jmcatamney/gpdb,yazun/postgres-xl,greenplum-db/gpdb,50wu/gpdb,50wu/gpdb,ashwinstar/gpdb,zeroae/postgres-xl,adam8157/gpdb,lisakowen/gpdb,jmcatamney/gpdb,techdragon/Postgres-XL,adam8157/gpdb,ovr/postgres-xl,xinzweb/gpdb,xinzweb/gpdb,oberstet/postgres-xl,jmcatamney/gpdb,adam8157/gpdb,adam8157/gpdb,pavanvd/postgres-xl,jmcatamney/gpdb,ovr/postgres-xl,lisakowen/gpdb,lisakowen/gpdb,xinzweb/gpdb,oberstet/postgres-xl,50wu/gpdb,lisakowen/gpdb,pavanvd/postgres-xl,greenplum-db/gpdb,oberstet/postgres-xl,greenplum-db/gpdb,jmcatamney/gpdb,adam8157/gpdb,greenplum-db/gpdb,Postgres-XL/Postgres-XL,yazun/postgres-xl,pavanvd/postgres-xl,pavanvd/postgres-xl,ovr/postgres-xl,Postgres-XL/Postgres-XL,techdragon/Postgres-XL,xinzweb/gpdb,xinzweb/gpdb,Postgres-XL/Postgres-XL,zeroae/postgres-xl,greenplum-db/gpdb,yazun/postgres-xl,Postgres-XL/Postgres-XL,yazun/postgres-xl,xinzweb/gpdb,techdragon/Postgres-XL,jmcatamney/gpdb,lisakowen/gpdb,yazun/postgres-xl,ashwinstar/gpdb,zeroae/postgres-xl,adam8157/gpdb,ashwinstar/gpdb,oberstet/postgres-xl,greenplum-db/gpdb,ashwinstar/gpdb,zeroae/postgres-xl,xinzweb/gpdb,50wu/gpdb,ovr/postgres-xl,zeroae/postgres-xl,adam8157/gpdb,xinzweb/gpdb,lisakowen/gpdb,techdragon/Postgres-XL,lisakowen/gpdb,lisakowen/gpdb,greenplum-db/gpdb,50wu/gpdb,pavanvd/postgres-xl,ashwinstar/gpdb,50wu/gpdb,jmcatamney/gpdb,ashwinstar/gpdb,greenplum-db/gpdb,oberstet/postgres-xl,50wu/gpdb