doc: make -- Add description for `obj-ext-src-y'

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

Documentation/Makefile.build.txt

@@ -61,6 +61,12 @@ obj-e::
 	Same as 'obj-y' but implies that source code file lays in directory
 	other than '$(obj)'. The postfix '-e' came from word 'external'.
 
+obj-ext-src-y::
+	Same as 'obj-y' but implies that source code file lays in directory
+	other than '$(obj)', while compiled object file pushed into '$(obj)' directory.
+	Consider using this variable if you need to compile same source file with
+	different flags.
+
 asm-y::
 	Source code S file. Same as 'obj-y' but for assembly language.