docs: note that --build may generate a non-functional profile

Users might expect `--build` to generate a complete and working profile
(see #6651), so clarify that this is not really the case.

src/man/firejail.1.in

@@ -226,6 +226,13 @@ The program is run in a very relaxed sandbox, with only \-\-caps.drop=all and
 Programs that raise user privileges are not supported.
 .br
 
+.br
+Note: This option is intended for profile debugging and development.
+The profile that is generated may be incomplete, non-functional and lacking in
+security.
+If you want to try to create a new profile, see CONTRIBUTING.md.
+.br
+
 .br
 Example:
 .br
@@ -240,6 +247,13 @@ The program is run in a very relaxed sandbox, with only \-\-caps.drop=all and
 Programs that raise user privileges are not supported.
 .br
 
+.br
+Note: This option is intended for profile debugging and development.
+The profile that is generated may be incomplete, non-functional and lacking in
+security.
+If you want to try to create a new profile, see CONTRIBUTING.md.
+.br
+
 .br
 Example:
 .br