LocalIP: document show-mtu and show-speed command line options (#1318)

Also add these options to shell completions.

completions/fastfetch.bash

@@ -185,6 +185,8 @@ __fastfetch_completion()
         "--localip-show-ipv6"
         "--localip-show-flags"
         "--localip-show-loop"
+        "--localip-show-mtu"
+        "--localip-show-speed"
         "--localip-name-prefix"
         "--localip-compact-type"
         "--escape-bedrock"

src/data/help.json

@@ -1196,6 +1196,24 @@
                 "default": false
             }
         },
+        {
+            "long": "localip-show-mtu",
+            "desc": "Show net interface's MTU (Maximum Transmission Unit) size in bytes",
+            "arg": {
+                "type": "bool",
+                "optional": true,
+                "default": false
+            }
+        },
+        {
+            "long": "localip-show-speed",
+            "desc": "Show net interface's link speed with human-readable format (or an 1Mb units integer in JSON format)",
+            "arg": {
+                "type": "bool",
+                "optional": true,
+                "default": false
+            }
+        },
         {
             "long": "localip-show-prefix-len",
             "desc": "Show network prefix length (/N) in local ip module",