doc/man1/openssl-enc.pod.in: document 'k' handling for -bufsize

doc/man1/openssl-enc.pod.in: document 'k' handling for -bufsize

Apparently, '-bufsize' option parser can handle the 'k' suffix
(and treat is as the value being provided in the multiples of 1024).
Document that.

Complements: d02b48c63a58 "Import of old SSLeay release: SSLeay 0.8.1b"
Signed-off-by: Eugene Syromiatnikov <esyr@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Paul Dale <ppzgs1@gmail.com>
Reviewed-by: Neil Horman <nhorman@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/28405)