search.h: Numerous grammatical fixes, comment updates

Tweaks (no functional changes) to include/search.h, including:

 * use standard multiple inclusion check
 * fix spelling mistakes
 * have comments match actual names in function prototypes
 * remove obsolete reference to "do_apply"
 * replace "hashing table" with "hash table"

Signed-off-by: Robert P. J. Day <rpjday@crashcourse.ca>
1 file changed