perldata: document binary/octal floating-point constants

This (possibly accidental) feature has had tests since commit
58be576 and bug fixes such as commit 2cb5a7e. See also the
discussion in GH issues #16114 and #14791, particularly jhi's comment in
<#16114 (comment)>:

> Relatedly: I remember there being a known "loophole" so that the
> scanning code currently accidentally, falling naturally out of the
> implementation, is also doing "binary fp" and "octal fp". Ah yes:
>
>     ./miniperl -wle 'print 0b11.1p0'
>     3.5
>     ./miniperl -wle 'print 011.1p0'
>     9.125
>
> This is probably not documented anywhere. I can't now think of the
> right search terms to find any previous discussion, there was something
> about should this be rejected, or not. If not (as is currently the
> case), maybe this possibly should be tested, documented, and made
> official?

With the documentation in this patch and the existing tests, I guess the
feature is now official.

Fixes #18664.

Author: mauke

pod/perldata.pod

@@ -444,6 +444,9 @@ integer formats:
  0o12_345            # alternative octal (introduced in Perl 5.33.5)
  0b011011            # binary
  0x1.999ap-4         # hexadecimal floating point (the 'p' is required)
+ 07.65p2             # octal floating point (the 'p' is required)
+ 0o7.65p2            # alternative octal floating point
+ 0b101.01p-1         # binary floating point (the 'p' is required)
 
 You are allowed to use underscores (underbars) in numeric literals
 between digits for legibility (but not multiple underscores in a row:
@@ -478,6 +481,9 @@ potential source of (low-order) differences are the floating point
 rounding modes, which can differ between CPUs, operating systems,
 and compilers, and which Perl doesn't control.
 
+Octal and binary floating point numbers use the same format as hexadecimal
+floating point numbers, but limited to binary and octal digits, respectively.
+
 You can also embed newlines directly in your strings, i.e., they can end
 on a different line than they begin.  This is nice, but if you forget
 your trailing quote, the error will not be reported until Perl finds