Changes to man page for SLK functions

[Bill Gray]

   Three (more) quite minor nits to pick.

(1) It appears to me that,  in the "synopsis" section of

https://invisible-island.net/ncurses/man/curs_slk.3x.html

   slk_attr() should be moved down a couple of lines to be under the /* 
extension */ section,  along with extended_slk_color().

(2) Much further down,  in the "portability" section,  it may be worth 
noting that the format codes 2 and 3 are ncurses extensions,  but are 
also available in PDCurses (and,  for that matter,  PDCursesMod).

(3) Near the end of that same section :

...extended versions of those functions which use short parameters...

   should say 'int parameters'.

-- Bill