doc: process get/setuid and get/setgid are POSIX only

Fixes #3302

doc/api/process.markdown

@@ -190,18 +190,25 @@ The shell that executed node should see the exit code as 1.
 
 ## process.getgid()
 
+Note: this function is only available on POSIX platforms (i.e. not Windows)
+
 Gets the group identity of the process. (See getgid(2).)
 This is the numerical group id, not the group name.
 
+    if (process.getgid) {
       console.log('Current gid: ' + process.getgid());
+    }
 
 
 ## process.setgid(id)
 
+Note: this function is only available on POSIX platforms (i.e. not Windows)
+
 Sets the group identity of the process. (See setgid(2).)  This accepts either
 a numerical ID or a groupname string. If a groupname is specified, this method
 blocks while resolving it to a numerical ID.
 
+    if (process.getgid && process.setgid) {
       console.log('Current gid: ' + process.getgid());
       try {
         process.setgid(501);
@@ -210,22 +217,30 @@ blocks while resolving it to a numerical ID.
       catch (err) {
         console.log('Failed to set gid: ' + err);
       }
+    }
 
 
 ## process.getuid()
 
+Note: this function is only available on POSIX platforms (i.e. not Windows)
+
 Gets the user identity of the process. (See getuid(2).)
 This is the numerical userid, not the username.
 
+    if (process.getuid) {
       console.log('Current uid: ' + process.getuid());
+    }
 
 
 ## process.setuid(id)
 
+Note: this function is only available on POSIX platforms (i.e. not Windows)
+
 Sets the user identity of the process. (See setuid(2).)  This accepts either
 a numerical ID or a username string.  If a username is specified, this method
 blocks while resolving it to a numerical ID.
 
+    if (process.getuid && process.setuid) {
       console.log('Current uid: ' + process.getuid());
       try {
         process.setuid(501);
@@ -234,6 +249,7 @@ blocks while resolving it to a numerical ID.
       catch (err) {
         console.log('Failed to set uid: ' + err);
       }
+    }
 
 
 ## process.version