res_pjsip: Add clarifying documentation to PJSIP_HEADER help text

Added notes about when you can read or write headers. Specifically
about being able to read on the inbound channel and write on an
outbound channel.

ASTERISK-26063 #close
Reported by: Private Name
Tested by: Rusty Newton

Change-Id: Ibeb64af17d1f6451028b3c29855a3f151a01d8c5

res/res_pjsip_header_funcs.c

@@ -39,7 +39,8 @@
 /*** DOCUMENTATION
 	<function name="PJSIP_HEADER" language="en_US">
 		<synopsis>
-			Gets, adds, updates or removes the specified SIP header from a PJSIP session.
+			Gets headers from an inbound PJSIP channel. Adds, updates or removes the
+			specified SIP header from an outbound PJSIP channel.
 		</synopsis>
 		<syntax>
 			<parameter name="action" required="true">
@@ -75,6 +76,10 @@
 
 		</syntax>
 		<description>
+			<para>PJSIP_HEADER allows you to read specific SIP headers from the inbound
+			PJSIP channel as well as write(add, update, remove) headers on the outbound
+			channel. One exception is that you can read headers that you have already
+			added on the outbound channel.</para>
 			<para>Examples:</para>
 			<para>;</para>
 			<para>; Set 'somevar' to the value of the 'From' header.</para>