Issue 4133: Give predicate ly:grob-properties? the description "grob properties"

Issue 4133: Give predicate ly:grob-properties? the description "grob properties"

    \set Staff.InstrumentName = "Test"

previously led to

    warning: type check for `InstrumentName' failed;
    value `"Test"' must be of type `ly:grob-properties'

and will now instead produce

    warning: type check for `InstrumentName' failed;
    value `"Test"' must be of type `grob properties'.

The output looks slightly less cryptic and more intentional.  The plural
form is indeed correct: this type of context property serves as a
container for _all_ grob properties of InstrumentName.