doc: add note to net.isIP() about limitations

Closes: nodejs#40966

Author: Trott

doc/api/net.md

@@ -1483,6 +1483,10 @@ Tests if input is an IP address. Returns `0` for invalid strings,
 returns `4` for IP version 4 addresses, and returns `6` for IP version 6
 addresses.
 
+`net.isIP()` validates conventional IP formats only. There are some
+technically permissible IP formats that it will reject. In practice, the formats
+that it rejects are not widely used except perhaps for malicious purposes.
+
 ## `net.isIPv4(input)`
 
 <!-- YAML
@@ -1494,6 +1498,10 @@ added: v0.3.0
 
 Returns `true` if input is a version 4 IP address, otherwise returns `false`.
 
+`net.isIPv4()` validates conventional IP formats only. There are some
+technically permissible IP formats that it will reject. In practice, the formats
+that it rejects are not widely used except perhaps for malicious purposes.
+
 ## `net.isIPv6(input)`
 
 <!-- YAML