config: move documentation to config.h

Move the documentation from Documentation/technical/api-config.txt into
config.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-config.txt is removed because the information
it has is now redundant and it'll be hard to keep it up to date and
syncronized with the documentation in config.h

Signed-off-by: Heba Waly <[email protected]>
Reviewed-by: Emily Shaffer <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: HebaWaly