Add IS [NOT] DISTINCT to boolean logic documentation

Add syntax to tables:

a IS DISTINCT FROM b:

• a or b is MISSING -> MISSING
• a is NULL and b is non-NULL -> TRUE
• a is non-NULL and b is NULL -> TRUE
• a is NULL and b is NULL -> FALSE
• a and b are non-NULL, non-MISSING then the result is the same as a != b

a IS NOT DISTINCT FROM b:

• a or b is MISSING -> MISSING
• a is NULL and b is non-NULL -> FALSE
• a is non-NULL and b is NULL -> FALSE
• a is NULL and b is NULL -> TRUE
• a and b are non-NULL, non-MISSING then the result is the same as a == b