Fix various typos in comments and documentation

Fix various typos in comments and documentation

* doc/README.Mac: Fix typo (replace "it's" to "its").
* doc/debugging.html: Likewise.
* doc/gcdescr.html: Fix typo ("it performs").
* include/private/gcconfig.h: Fix typo in comment ("its").
* mark.c (INITIAL_MARK_STACK_SIZE): Fix typo in comment ("it wants").
* mark.c (GC_mark_from): Fix typo in comment (double "it").
* misc.c (GC_SLOP): Fix typo in comma (double "saw").
* os_dep.c (GC_get_file_len): Fix typo in comment (missing "buffer").
* ptr_chck.c (GC_is_visible): Add missing comma in comment.