Add more thorough documentation for external integration

This is the first step in adding easier support for external statusline
integrations.
In the future we can add more examples for programs such as zellij,
WM statuslines, etc...

These docs always should go into separate files so that the main
documentation doesn't get bloated.

We could also think about adding an extra configuration variable that
can toggle between some popular external statusline examples.

Author: vimpostor

doc/tags

@@ -22,6 +22,10 @@ tpipeline#update()	tpipeline.txt	/*tpipeline#update()*
 tpipeline-api	tpipeline.txt	/*tpipeline-api*
 tpipeline-configure	tpipeline.txt	/*tpipeline-configure*
 tpipeline-contents	tpipeline.txt	/*tpipeline-contents*
+tpipeline-external	tpipeline-external.txt	/*tpipeline-external*
+tpipeline-external-examples	tpipeline-external.txt	/*tpipeline-external-examples*
+tpipeline-external-introduction	tpipeline-external.txt	/*tpipeline-external-introduction*
+tpipeline-external.txt	tpipeline-external.txt	/*tpipeline-external.txt*
 tpipeline-introduction	tpipeline.txt	/*tpipeline-introduction*
 tpipeline-kitty	tpipeline-kitty.txt	/*tpipeline-kitty*
 tpipeline-kitty-configure	tpipeline-kitty.txt	/*tpipeline-kitty-configure*

doc/tpipeline-external.txt

@@ -0,0 +1,31 @@
+*tpipeline-external.txt*	External statusline integration
+*tpipeline-external*
+
+
+Introduction					|tpipeline-external-introduction|
+Examples					|tpipeline-external-examples|
+
+==============================================================================
+INTRODUCTION						*tpipeline-external-introduction*
+
+This plugin is not limited to embedding the statusline in tmux only. The
+statusline can be embedded in arbitrary external programs. The
+|g:tpipeline_refreshcmd| option already allows running arbitrary commands to
+trigger the update of an external statusline.
+
+Depending on the external program, some integration needs to be done on that
+side as well. There exists full documentation for some external programs in
+|tpipeline-external-examples|.
+
+------------------------------------------------------------------------------
+EXAMPLES						*tpipeline-external-examples*
+
+The following list contains documentation for examples of embedding the
+statusline in some popular external programs that are not tmux:
+
+	Kitty statusline				|tpipeline-kitty|
+		Embeds the vim statusline in the Kitty terminal emulator
+		statusline.
+
+==============================================================================
+	vim:tw=78:sw=4:ts=8:ft=help:norl:noet:

doc/tpipeline.txt

@@ -19,6 +19,9 @@ get a total of two extra lines of buffer content available.
 This plugin also comes with its own statusline that you can use independently
 of the tmux functionality.
 
+Embedding the statusline is not just limited to tmux but can be made to work
+with arbitrary other programs, see |tpipeline-external|.
+
 ------------------------------------------------------------------------------
 TMUX INTEGRATION					*tpipeline-tmux*
 
@@ -126,8 +129,10 @@ CONFIGURATION						*tpipeline-configure*
 		If you want to run an external command every time the
 		statusline changes, you can provide a shell command with this
 		variable.
+		The provided command is run from a separate process and never
+		blocks vim.
 		This can be used for external statusline integrations
-		different from tmux.
+		different from tmux, see |tpipeline-external|.
 
 	g:tpipeline_clearstl				*g:tpipeline_clearstl*
 		If you want this plugin to clear the vim internal statusline