Bug 1602105: Mention default value of Span<T, Extent>'s extent; r=perry

MozReview-Commit-ID: 5h8zGik856o

Differential Revision: https://phabricator.services.mozilla.com/D56242

--HG--
extra : moz-landing-system : lando

mfbt/Span.h

@@ -358,7 +358,7 @@ class extent_type<dynamic_extent> {
  * AsBytes(). Any Span<T> can be viewed as Span<uint8_t> using the function
  * AsWritableBytes().
  */
-template <class ElementType, size_t Extent>
+template <class ElementType, size_t Extent /* = dynamic_extent */>
 class Span {
  public:
   // constants and types