multi-wc-format: user visibility of WC version (info --show-item)

Daniel Shahaf wrote, in dev@ email "multi-wc-format review":

1. How can a user ask a working copy what range of minor versions it
supports? Cf. "Compatible With Version:" in `svnadmin info` output.

I see these:

% subversion/svn/svn info wc --show-item=wc-format
31
% subversion/svn/svn info wc --show-item=wc-format-min
31
% subversion/svn/svn info wc --show-item=wc-format-max
32
%

but:

• They emit format numbers rather than version numbers. It would be
  more useful to emit version numbers, or at least to expose the
  format-number-to-version-number mapping in the public API.

• They aren't documented in the release notes (only in the help output).

• wc-format-min and wc-format-max are not attributes of the working
  copy but of the Subversion library doing the query. It's this way
  even in the API (svn_wc_info_t::wc_format_min and
  svn_wc_info_t::wc_format_max), but shouldn't we change this? "Do one
  thing and do it well.". «svn info»'s one thing is to report properties
  of the working copy. Information about the library's capabilities
  clearly belongs in «svn --version --xml»

[«svn --version» does in fact print the library's supported version
number range already.]