Document how out-of-range inputs are handled

This changed recently so that overflow and underflow consistently return signed
infinity or zero instead of nil.  (Previously, overflow returned nil, underflow
returned zero.)

Among other benefits:
 * The new behavior distinguishes malformed input (nil) from valid but out-of-range input.
 * The new behavior preserves the sign of the input
 * The new behavior is consistent with how floating-point parsing behaves in other langauges

Resolves rdar://76728925

Author: tbkka

stdlib/public/core/FloatingPointParsing.swift.gyb

@@ -128,8 +128,10 @@ extension ${Self}: LosslessStringConvertible {
   ///     ${Self}("0x1.25e4")  // Incorrect exponent format
   ///
   /// - Parameter text: The input string to convert to a `${Self}` instance. If
-  ///   `text` has invalid characters or is in an invalid format, the result
-  ///   is `nil`.
+  ///   `text` has invalid characters or is in an invalid format, the result is
+  ///   `nil`.  If the input is valid but the parsed value would be too small
+  ///   or large for the type, the result will be a zero or infinity with the
+  ///   appropriate sign.
   @inlinable
   public init?<S: StringProtocol>(_ text: S) {
   %if bits == 16: