cmd/godoc: cleanup documentation for CLI mode

- Removed all documentation which referred to godoc's CLI capabilities.
- Added some missing flags which were not documented.
- Removed some redundant mentions of "web server" because now it is
just a web server.
- Converted some links to https

Change-Id: I7ddcf417d2bc3df13183adcfd0f82af9a37a1b91
Reviewed-on: https://go-review.googlesource.com/c/143077
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
diff --git a/godoc/server.go b/godoc/server.go
index db07c99..57766a2 100644
--- a/godoc/server.go
+++ b/godoc/server.go
@@ -208,6 +208,7 @@
 		timestamp = ts
 	}
 	if dir == nil {
+		// TODO(agnivade): handle this case better, now since there is no CLI mode.
 		// no directory tree present (happens in command-line mode);
 		// compute 2 levels for this page. The second level is to
 		// get the synopses of sub-directories.