runtime(doc): rewrite mkdir() doc and simplify {flags} meaning

related: #14991

Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/runtime/doc/builtin.txt b/runtime/doc/builtin.txt
index dd46430..862a1df 100644
--- a/runtime/doc/builtin.txt
+++ b/runtime/doc/builtin.txt
@@ -7329,17 +7329,14 @@
 		When {flags} is present it must be a string.  An empty string
 		has no effect.
 
-		If {flags} contains "p" then intermediate directories are
-		created as necessary.
+		{flags} can contain these character flags:
+		 "p"	intermediate directories will be created as necessary
+		 "D"	{name} will be deleted at the end of the current
+			function, but not recursively |defer|
+		 "R"	{name} will be deleted recursively at the end of the
+			current function |defer|
 
-		If {flags} contains "D" then {name} is deleted at the end of
-		the current function, as with: >
-			defer delete({name}, 'd')
-<
-		If {flags} contains "R" then {name} is deleted recursively at
-		the end of the current function, as with: >
-			defer delete({name}, 'rf')
-<		Note that when {name} has more than one part and "p" is used
+		Note that when {name} has more than one part and "p" is used
 		some directories may already exist.  Only the first one that
 		is created and what it contains is scheduled to be deleted.
 		E.g. when using: >