VSIPLXX12 — "this header is usually included" should be excluded

These sections contain phrases like, "Note: This header file is usually
included in other header files so direct inclusion is rarely necessary."

This is non-normative, and because "usually" means "some implementations
may not do this", it is not helpful to users in writing
standard-conformant code. In fact, it encourages users to write
non-comforming code! These should be removed.

The following note has been removed from item 4/1 (page 11):
“[Note: This header file is usually included in other header files so direct inclusion
is rarely necessary.]”