attr: move doc to attr.h

Move the documentation from Documentation/technical/api-gitattributes.txt
to attr.h as it's easier for the developers to find the usage
information beside the code instead of looking for it in another doc file.

Also documentation/technical/api-gitattributes.txt is removed because the
information it has is now redundant and it'll be hard to keep it up to
date and synchronized with the documentation in the header file.

Signed-off-by: Heba Waly <heba.waly@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

attr.c

@@ -1,7 +1,6 @@
 /*
  * Handle git attributes.  See gitattributes(5) for a description of
- * the file syntax, and Documentation/technical/api-gitattributes.txt
- * for a description of the API.
+ * the file syntax, and attr.h for a description of the API.
  *
  * One basic design decision here is that we are not going to support
  * an insanely large number of attributes.