ELF: document some de-facto PT_* ABI quirks

Turns out rules about PT_INTERP, PT_GNU_STACK and PT_GNU_PROPERTY
program headers are slightly different.

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Link: https://lore.kernel.org/r/88d3f1bb-f4e0-4c40-9304-3843513a1262@p183


Signed-off-by: Jonathan Corbet <corbet@lwn.net>