Define a workaround for empty \retval description

Since Clang 15, `clang -Wdocumentation` warns about an empty description in
a Doxygen `\retval` command:

```
include/psa/crypto.h:91:23: error: empty paragraph passed to '\retval' command [-Werror,-Wdocumentation]
 * \retval #PSA_SUCCESS
   ~~~~~~~~~~~~~~~~~~~^
```

Ideally `\retval` directives should have a description that describes the
precise meaning of the return value, but we commonly use an empty
description when the return value is a status code and the status code's
description is sufficient documentation.

As a workaround, define a Doxygen command `\emptydescription` that we can
use to make the description source code non-empty, without changing the
appearance. Using the command will be done in a subsequent commit.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>

doxygen/mbedtls.doxyfile

@@ -25,3 +25,15 @@ HAVE_DOT               = YES
 DOT_GRAPH_MAX_NODES    = 200
 MAX_DOT_GRAPH_DEPTH    = 1000
 DOT_TRANSPARENT        = YES
+
+# Doxygen accepts empty descriptions for commands such as \retval,
+# but clang -Wdocumentation doesn't (since Clang 15, for \retval).
+#   https://github.com/Mbed-TLS/mbedtls/issues/6960
+#   https://github.com/llvm/llvm-project/issues/60315
+# As a workaround, when documenting the status codes that a function can
+# return, if you don't have anything to say beyond the status code's
+# description, you can write something like
+#    \retval #PSA_ERROR_INSUFFICIENT_MEMORY \emptydescription
+# This does not change the documentation generated by Doxygen, but
+# it pacifies clang -Wdocumentation.
+ALIASES += emptydescription=""