(Output Variables): Doc float-output-format.

lispref/streams.texi

@@ -821,6 +821,17 @@ the @code{print-circle} feature.  You should not use it except
 to bind it to @code{nil} when you bind @code{print-continuous-numbering}.
 @end defvar
 
+@defvar float-output-format
+This variable specifies how to print floating point numbers.  Its
+default value is @code{nil}, meaning use the shortest output
+that represents the number without losing information.
+
+To control output format more precisely, you can put a string in this
+variable.  The string should hold a @samp{%}-specification to be used
+in the C function @code{sprintf}.  For further restrictions on what
+you can use, see the variable's documentation string.
+@end defvar
+
 @ignore
    arch-tag: 07636b8c-c4e3-4735-9e06-2e864320b434
 @end ignore