DOC: configuration: explain the rules regarding spaces in arguments

Spaces around commas or parenthesis in expressions are generally part
of the value due to the long history of supporting unquoted arguments.
But this tends to come as a surprise to new users and sometimes creates
subtly invalid configurations. Let's add some text covering this.

This can be backported to 2.4.

Author: wtarreau

doc/configuration.txt

@@ -852,6 +852,26 @@ double quotes inside (so that the double quotes are not stripped again):
                                           arg2 ___________/    /
                                           arg3 _______________/
 
+But in this case it's important to note that delimiters embedded into the
+higher level string remain pure characters and are not delimiters anymore. It
+particularly means that spaces and tabs around commas are part of the string.
+The example below is wrong on multiple points:
+
+    http-request set-path '%[path, regsub("(foo|bar)", blah, g)]'
+    ------------ --------  --------------------------------------
+       word1       word2    word3=%[path, regsub("(foo|bar)", blah, g)]
+                                        |--------|---------||-----|--|
+                       converter=" regsub" _/        /         /   /
+                                    arg1=(foo|bar) _/         /   /
+                                     arg2=" blah" ___________/   /
+                                        arg3=" g" ______________/
+
+The single fact of surrounding commas with spaces resulted in the spaces being
+part of the field itself, hence the converter " regsub" (starting with a
+space), which won't be found and will trigger an error, but more subtly, the
+replacement string " blah" will insert a space in the output. A good rule of
+thumb is to never insert unneeded spaces inside expressions.
+
 When using regular expressions, it can happen that the dollar ('$') character
 appears in the expression or that a backslash ('\') is used in the replacement
 string. In this case these ones will also be processed inside the double quotes