[stdlib][docs] Clarify BidirectionalCollection requirements

As discussed on the forum, `BidirectionalCollection` requirements as
currently stated can be interpreted to forbid conforming types from
accepting indices that lie between valid (i.e., reachable) indices
in the collection. Among other undesirable effects, this
interpretation would render SE-0180 (String.Index overhaul)
incompatible with these requirements.

Update the wording to clarify that the requirements only apply to
valid indices. (Collection protocols do not constrain the behavior of
invalid indices — it’s up to individual collection types to implement
them as they wish.)

https://forums.swift.org/t/string-index-unification-vs-bidirectionalcollection-requirements/55946
rdar://92297280

Author: lorentey

stdlib/public/core/BidirectionalCollection.swift

@@ -28,13 +28,17 @@
 /// `Collection` protocol.
 ///
 /// Indices that are moved forward and backward in a bidirectional collection
-/// move by the same amount in each direction. That is, for any index `i` into
-/// a bidirectional collection `c`:
+/// move by the same amount in each direction. That is, for any valid index `i`
+/// into a bidirectional collection `c`:
 ///
 /// - If `i >= c.startIndex && i < c.endIndex`, then
 ///   `c.index(before: c.index(after: i)) == i`.
 /// - If `i > c.startIndex && i <= c.endIndex`, then
 ///   `c.index(after: c.index(before: i)) == i`.
+///
+/// Valid indices are exactly those indices that are reachable from the
+/// collection's `startIndex` by repeated applications of `index(after:)`, up
+/// to, and including, the `endIndex`.
 public protocol BidirectionalCollection: Collection
 where SubSequence: BidirectionalCollection, Indices: BidirectionalCollection {
   // FIXME: Only needed for associated type inference.