remove claim of XSI coverage from README

in addition to the dbm functions (which we don't intent to implement
anyway), fmtmsg is still missing too. rather than adding exceptions I
think it's best just to avoid making the claim.

README

@@ -13,8 +13,7 @@ achieved through simple code that is easy to understand and maintain.
 The 1.0 release series for musl features coverage for all interfaces
 defined in ISO C99 and POSIX 2008 base, along with a number of
 non-standardized interfaces for compatibility with Linux, BSD, and
-glibc functionality. Also covered are all XSI interfaces except the
-dbm functions, which are traditionally a separate library from libc.
+glibc functionality.
 
 For basic installation instructions, see the included INSTALL file.
 Information on full musl-targeted compiler toolchains, system