patch 8.2.0578: heredoc for interfaces does not support "trim"

Problem:    Heredoc for interfaces does not support "trim".
Solution:   Update the script heredoc support to be same as the :let command.
            (Yegappan Lakshmanan, closes #5916)
diff --git a/runtime/doc/if_pyth.txt b/runtime/doc/if_pyth.txt
index 88e0df6..572e6bf 100644
--- a/runtime/doc/if_pyth.txt
+++ b/runtime/doc/if_pyth.txt
@@ -34,7 +34,7 @@
 			the `:python` command is working: >
 				:python print "Hello"
 
-:[range]py[thon] << [endmarker]
+:[range]py[thon] << [trim] [{endmarker}]
 {script}
 {endmarker}
 			Execute Python script {script}.
@@ -42,10 +42,9 @@
 			feature wasn't compiled in.  To avoid errors, see
 			|script-here|.
 
-The {endmarker} below the {script} must NOT be preceded by any white space.
-
 If [endmarker] is omitted from after the "<<", a dot '.' must be used after
-{script}, like for the |:append| and |:insert| commands.
+{script}, like for the |:append| and |:insert| commands.  Refer to
+|:let-heredoc| for more information.
 
 This form of the |:python| command is mainly useful for including python code
 in Vim scripts.
@@ -768,12 +767,12 @@
 
 							*:py3* *:python3*
 :[range]py3 {stmt}
-:[range]py3 << [endmarker]
+:[range]py3 << [trim] [{endmarker}]
 {script}
 {endmarker}
 
 :[range]python3 {stmt}
-:[range]python3 << [endmarker]
+:[range]python3 << [trim] [{endmarker}]
 {script}
 {endmarker}
 	The `:py3` and `:python3` commands work similar to `:python`.  A