Document the two senses of "index"

The audit flagged NearestNeighborSearchIndex as misnamed on the grounds
that it holds List<Index> and List<Cache>, so it is the engine rather than
an index. That reasoning missed that the word covers two levels and both
are right in their place: the library as a whole is an index, which is the
sense Uber Similarity Search Index carries, while an Index is one
searchable structure inside it.

So the facade keeps its name, and the actual confusion - one word, two
levels - is stated where it belongs rather than fixed by a rename that
would leave a library called an index with nothing named one at the top.

Author: drametwally

DESIGN.md

@@ -127,6 +127,23 @@ Enums a config names implement `ConfigVocabulary`, which holds the one
 case-insensitive lookup and the one unsupported-value message. A config never
 names a record type, so `RecordType` is not among them.
 
+## Two Senses Of "Index"
+
+The word covers two levels, and both are correct in their place.
+
+The library is an index: `NearestNeighborSearchIndex` is the whole searchable
+thing, which is the sense the project's own name carries in Uber Similarity
+Search Index. It stores rows, answers queries, and is the only type an
+embedder needs.
+
+An `Index` is one searchable structure inside it, a sibling of a `Cache` and
+a `SearchableStructure` like it. The facade owns a list of these and a list
+of caches, and merges their results.
+
+So the facade holding indexes rather than being one is not a contradiction.
+A reader who expects `NearestNeighborSearchIndex` to extend `Index` has the
+narrow sense in mind; nothing does, and nothing should.
+
 ## Package Layout
 
 ### Searchable Structures