libmisc/salt.c: Add comments how the minmum buffer length is computed.

libmisc/salt.c: Add comments how the minmum buffer length is computed.

In the previous commit we refactored the functions converting the
rounds number into a string for use with the crypt() function, to
not require any static buffer anymore.

Add some clarifying comments about how the minimum required buffer
length is computed inside of these functions.

Signed-off-by: Björn Esser <besser82@fedoraproject.org>