Document `IRODS_DATABASE_USER_PASSWORD_SALT`

[korydraughn]

Motivated by https://groups.google.com/g/irod-chat/c/4BEdYS71ADI/m/ELRAFUEwEAAJ?utm_medium=email&utm_source=footer.

[korydraughn]

Here are all of the locations in irods/irods that reference IRODS_DATABASE_USER_PASSWORD_SALT.

• https://github.com/irods/irods/blob/69874094328aba0667d0b56156a6483b4d3668c6/plugins/database/src/db_plugin.cpp#L110
• https://github.com/irods/irods/blob/69874094328aba0667d0b56156a6483b4d3668c6/scripts/irods/database_interface.py#L202
• https://github.com/irods/irods/blob/69874094328aba0667d0b56156a6483b4d3668c6/scripts/irods/database_connect.py#L416