Document default value for maximum_bits.

doc/language-reference.md

diff --git a/doc/language-reference.md b/doc/language-reference.md
index ed6ecc0..7eda923 100644
--- a/doc/language-reference.md
+++ b/doc/language-reference.md
@@ -782,6 +782,8 @@
   #4 [+8]  ExplicitlySized  eight_bytes  # 64-bit field would be an error
 ```
 
+If not specified, `maximum_bits` defaults to `64`.
+
 This also allows back end code generators to use smaller types for `enum`s, in
 some cases.