Fix incorrect and inconsistent comments in tableam.h and heapam.c.

Fix incorrect and inconsistent comments in tableam.h and heapam.c.

This commit corrects several issues in function comments:

* The parameter "rel" was incorrectly referred to as "relation" in the comments
   for table_tuple_delete(), table_tuple_update(), and table_tuple_lock().
* In table_tuple_delete(), "changingPart" was listed as an output parameter
   in the comments but is actually input.
* In table_tuple_update(), "slot" was listed as an input parameter
   in the comments but is actually output.
* The comment for "update_indexes" in table_tuple_update() was mis-indented.
* The comments for heap_lock_tuple() incorrectly referenced a non-existent
   "tid" parameter.

Author: Chao Li <lic@highgo.com>
Reviewed-by: Fujii Masao <masao.fujii@gmail.com>
Discussion: https://postgr.es/m/CAEoWx2nB6Ay8g=KEn7L3qbYX_4+sLk9XOMkV0XZqHR4cTY8ZvQ@mail.gmail.com