sigchain: move doc to sigchain.h

Move the documentation from Documentation/technical/api-sigchain.txt
to sigchain.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-sigchain.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>

Documentation/technical/api-sigchain.txt

@@ -1,41 +0,0 @@
-sigchain API
-============
-
-Code often wants to set a signal handler to clean up temporary files or
-other work-in-progress when we die unexpectedly. For multiple pieces of
-code to do this without conflicting, each piece of code must remember
-the old value of the handler and restore it either when:
-
-  1. The work-in-progress is finished, and the handler is no longer
-     necessary. The handler should revert to the original behavior
-     (either another handler, SIG_DFL, or SIG_IGN).
-
-  2. The signal is received. We should then do our cleanup, then chain
-     to the next handler (or die if it is SIG_DFL).
-
-Sigchain is a tiny library for keeping a stack of handlers. Your handler
-and installation code should look something like:
-
-------------------------------------------
-  void clean_foo_on_signal(int sig)
-  {
-	  clean_foo();
-	  sigchain_pop(sig);
-	  raise(sig);
-  }
-
-  void other_func()
-  {
-	  sigchain_push_common(clean_foo_on_signal);
-	  mess_up_foo();
-	  clean_foo();
-  }
-------------------------------------------
-
-Handlers are given the typedef of sigchain_fun. This is the same type
-that is given to signal() or sigaction(). It is perfectly reasonable to
-push SIG_DFL or SIG_IGN onto the stack.
-
-You can sigchain_push and sigchain_pop individual signals. For
-convenience, sigchain_push_common will push the handler onto the stack
-for many common signals.