Make it obvious that host and port are both optional (#204)

* doc: make it obvious that host and port are both optional

* fix minor issue

* address comments

README.md

 
 ## Run pprof via a web interface
 
-If the `-http="host:port"` option is specified, pprof starts a web server at
+If the `-http` flag is specified, pprof starts a web server at
 the specified host:port that provides an interactive web-based interface to pprof.
+Host is optional, and is "localhost" by default. Port is optional, and is a
+random available port by default. `-http=":"` starts a server locally at
+a random port.
 
 ```
-pprof -http=[host:port] [main_binary] profile.pb.gz
+pprof -http=[host]:[port] [main_binary] profile.pb.gz
 ```
 
 The preceding command should automatically open your web browser at

doc/pprof.md

 
 ## Web interface
 
-If a port is specified on the command line:
+If a host:port is specified on the command line:
 
-    pprof -http=<port> [options] source
+    pprof -http=[host]:[port] [options] source
 
 pprof will start serving HTTP requests on the specified port.  Visit
-the HTTP url corresponding to the port (typically `http://localhost:<port>/`)
+the HTTP url corresponding to the port (typically `http://<host>:<port>/`)
 in a browser to see the interface.
 
 # Details

internal/driver/cli.go

 		return nil, nil, err
 	}
 	if cmd != nil && *flagHTTP != "" {
-		return nil, nil, fmt.Errorf("--http is not compatible with an output format on the command line")
+		return nil, nil, fmt.Errorf("-http is not compatible with an output format on the command line")
 	}
 
 	si := pprofVariables["sample_index"].value
 interface at the specified host:port that can be used to navigate through
 various views of a profile.
 
-   pprof -http <host:port> [options] [binary] <source> ...
+   pprof -http [host]:[port] [options] [binary] <source> ...
 
 Details:
 `
 
 var usageMsgVars = "\n\n" +
 	"  Misc options:\n" +
-	"   -http host:port        Provide web based interface at host:port\n" +
-	"   -tools                 Search path for object tools\n" +
+	"   -http              Provide web based interface at host:port.\n" +
+	"                      Host is optional and 'localhost' by default.\n" +
+	"                      Port is optional and a randomly available port by default.\n" +
+	"   -tools             Search path for object tools\n" +
 	"\n" +
 	"  Legacy convenience options:\n" +
 	"   -inuse_space           Same as -sample_index=inuse_space\n" +