CodingGuidelines: mention -Wunused-parameter and UNUSED

Now that -Wunused-parameter is on by default for DEVELOPER=1 builds,
people may trigger it, blocking their build. When it's a mistake for the
parameter to exist, the path forward is obvious: remove it. But
sometimes you need to suppress the warning, and the "UNUSED" mechanism
for that is specific to our project, so people may not know about it.

Let's put some advice in CodingGuidelines, including an example warning
message. That should help people who grep for the warning text after
seeing it from the compiler.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/CodingGuidelines

@@ -258,6 +258,13 @@ For C programs:
    ensure your patch is clear of all compiler warnings we care about,
    by e.g. "echo DEVELOPER=1 >>config.mak".
 
+ - When using DEVELOPER=1 mode, you may see warnings from the compiler
+   like "error: unused parameter 'foo' [-Werror=unused-parameter]",
+   which indicates that a function ignores its argument. If the unused
+   parameter can't be removed (e.g., because the function is used as a
+   callback and has to match a certain interface), you can annotate the
+   individual parameters with the UNUSED keyword, like "int foo UNUSED".
+
  - We try to support a wide range of C compilers to compile Git with,
    including old ones.  As of Git v2.35.0 Git requires C99 (we check
    "__STDC_VERSION__"). You should not use features from a newer C