Hide C compiler warnings about unused items, explain some others

These are numerous, but harmless -- that's just how Vala → C generation
works. It takes attention away from the more interesting warnings.

Add explanation and bug reference for the others.

Fixes #226

meson.build

@@ -39,14 +39,19 @@ g_ir_compiler = find_program('g-ir-compiler', required: false)
 # generated vala code triggers tons of these
 add_project_arguments(
   cc.get_supported_arguments(
+    # Vala doesn't have `const void*`, or a way to selectively #pragma these in the code
     '-Wno-error=discarded-qualifiers',
+    # mostly const dropping and dropped "volatile" in fully autogenerated code;
+    # upstream Vala does not support fixing this warning
     '-Wno-error=incompatible-pointer-types',
+    # https://gitlab.gnome.org/GNOME/vala/-/issues/1529
     '-Wno-error=pointer-sign',
-    '-Wno-error=unused-but-set-variable',
-    '-Wno-error=unused-function',
-    '-Wno-error=unused-label',
-    '-Wno-error=unused-value',
-    '-Wno-error=unused-variable'
+    # "mostly harmless", fallout from Vala C code template generation, C compiler will optimize away
+    '-Wno-unused-but-set-variable',
+    '-Wno-unused-function',
+    '-Wno-unused-label',
+    '-Wno-unused-value',
+    '-Wno-unused-variable'
   ), language: 'c')
 
 # HACK: vala doesn't support declaring const arguments, so we get a lot of errors