Clearly state that custom subs applies to any block

[mojavelinux]

Clearly state that custom substitutions, declared using the subs attribute, applies to any block or paragraph.

Also clarify that, as of 1.5.0, if the first entry in the list is not preceded by a + or -, then the only substitutions applied are the ones listed. The preceding + or - reflects a modification to the built-in subs for that block.