set-text-properties, remove-set-properties, add-text-properties: 1) doc string 2) return value

[Drew Adams]

1. The set-text-properties doc string should say what the function returns.

2. Wouldn't it be more useful for set-text-properties,
remove-set-properties, and add-text-properties to return the modified OBJECT
(or nil if no modification occurred)? In case OBJECT is a buffer (or nil),
the modified buffer substring could be returned.

Perhaps it's too late to make the change in #2, if existing code depends on
the current value - but maybe not:

 - In the case of set-text-properties, that value is not even documented.

 - In the case of remove-set-properties and add-text-properties, perhaps
existing code only tests whether the returned value is null (instead of
testing equality with t).