Documentation improvement suggestion: << >>

Hello, Lilypond!

I've transcribed my first simple piece, and now I'm trying to learn some
more sophisticated features, and I ran into strange new syntax in many of
the examples: << >>

In order to figure out what this means, here's what I tried:
 - Looked in the symbol section of the index (it's not there):
https://lilypond.org/doc/v2.25/Documentation/notation/lilypond-index
 - Used the search box on the documentation page (which uses google.com,
which is not so good at searching for non-alphanumeric symbols)
 - Tried searching these same places for descriptions and synonyms, like
"less than"
 - Tried searching for other grouping symbols (like {} and []) which I
thought might be synonyms (but aren't)
 - Tried a general web search for all of the above, too
 - Downloaded the Lilypond notation reference as a PDF (930 pages!), and
tried a full-text search for the string "<<" (450 matches!)

Eventually, I ran across the description of this syntax, purely by luck:
it's a "temporary polyphonic passage" --
https://lilypond.org/doc/v2.24/Documentation/notation/multiple-voices#single_002dstaff-polyphony

The notation reference is huge but that's OK because it's generally good at
letting me learn syntax on an as-needed basis.  But in the case of << >>
I'm not sure how I could have looked this up, short of reading the entire
manual cover-to-cover.  The reference manual uses << >> in examples 73
times before finally describing (on page 213) what it means.

I guess this is a long-winded way of saying that I think << >> should be in
the index.

Thanks for a great program!


- Ken