Pacify Clang >=15 which complained:
```
include/psa/crypto.h:91:23: error: empty paragraph passed to '\retval' command [-Werror,-Wdocumentation]
* \retval #PSA_SUCCESS
~~~~~~~~~~~~~~~~~~~^
```
This commit performs the following systematic replacement:
```
perl -i -0777 -p -e 's/([\\@])(retval +\S+)\n(?! *\*? *([^\n \\*\/]|\\[cp]\b))/$1$2 ${1}emptydescription\n/g' $(git ls-files '*.[hc]' '*.function' '*.jinja')
```
i.e. add an `\emptydescription` argument to `\retval` commands (or
`@retval`, which we don't normally used) that are followed by a single word,
unless the next line looks like it contains text which would be the
description.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
ed73355d2e