doc: Clarify size callback

It is not correct that it is called on every statusline evaluation.
It's only called heuristically, e.g. when starting up or when the vim
window size changes.

If the users want to update the size at their own rate, they can still
just set g:tpipeline_size directly.

Author: vimpostor

doc/tpipeline.txt

@@ -170,9 +170,8 @@ CONFIGURATION						*tpipeline-configure*
 		By default the statusline size is calculated automatically to
 		span the entire tmux window size. With this option you can
 		overwrite a custom statusline size to be used. If the size is
-		not constant and should be recalculated on every statusline
-		evaluation, then the |User_TpipelineSize| autocommand can be
-		used.
+		not constant and should be recalculated on demand, then it can
+		be set from the |User_TpipelineSize| autocommand.
 
 ------------------------------------------------------------------------------
 API							*tpipeline-api*