doco cross references

[Kevin Ryde]

In the "Time" node of the guile reference manual, the cross reference
to libc strftime looks like it should be "Formatting Calendar Time"
these days (eg. Debian packaged glibc 2.3.1), rather than "Formatting
Date and Time".

In the "Macros guile-snarf recognizes" node of the reference manual,
the cross reference for goops is "(goops)GOOPS", but there seems to be
no such node.  Maybe it should be something like

        @pxref{Creating Generic Functions,,, goops, GOOPS}

In the "What is libguile" node of the guile tutorial, the cross
reference to the reference manual comes out as "*Note Guile Reference
Manual: (Guile Reference Manual)Scheme data representation." which
clearly isn't a correct filename, and the node name seems to be out of
date too.  Perhaps,

        @ref{Data representation,, Data Representation in Guile,
        guile, The Guile Reference Manual}