Do not touch sys.path when site is imported and python was started with -S.

Original patch by Carl Meyer, review by Brett Cannon, small doc editions by
yours truly.  Fixes #11591.
diff --git a/Misc/python.man b/Misc/python.man
index 2d15d5d..53b77fc 100644
--- a/Misc/python.man
+++ b/Misc/python.man
@@ -169,7 +169,9 @@
 .I site
 and the site-dependent manipulations of
 .I sys.path
-that it entails.
+that it entails.  Also disable these manipulations if
+.I site
+is explicitly imported later.
 .TP
 .B \-u
 Force the binary I/O layers of stdin, stdout and stderr to be unbuffered.