doc/man3/RAND_load_file.pod: RAND_load_file on non-regular files with bytes=-1

doc/man3/RAND_load_file.pod: RAND_load_file on non-regular files with bytes=-1

Mention that RAND_load_file attempts to read only RAND_DRBG_STRENGTH
bytes on non-regular files if the number of bytes to be read
is not specified explicitly.

Signed-off-by: Eugene Syromiatnikov <esyr@openssl.org>
Reviewed-by: Paul Dale <ppzgs1@gmail.com>
Reviewed-by: Saša Nedvědický <sashan@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/28379)