api-parse-options.txt: document OPT_CMDMODE()

OPT_CMDMODE mechanism was introduced in the release of 1.8.5 to actively
notice when multiple "operation mode" options that specify mutually
incompatible operation modes are given.

Signed-off-by: Pranit Bauva <pranit.bauva@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: pranitbauva1997

Documentation/technical/api-parse-options.txt

@@ -231,6 +231,13 @@ There are some macros to easily define options:
 	pass the command-line option, which can be specified multiple times,
 	to another command.
 
+`OPT_CMDMODE(short, long, &int_var, description, enum_val)`::
+	Define an "operation mode" option, only one of which in the same
+	group of "operating mode" options that share the same `int_var`
+	can be given by the user. `enum_val` is set to `int_var` when the
+	option is used, but an error is reported if other "operating mode"
+	option has already set its value to the same `int_var`.
+
 
 The last element of the array must be `OPT_END()`.