feat!: use XDG_DATA_HOME as AGENDA_DIR fallback

I first decided against this, but the old `~/.app-trash` is no more
fitting than the XDG_DATA_HOME location.

The xdg-user-dir DOCUMENTS fallback is too presumptuous and removing it
makes things even simpler.

agenda.1

@@ -107,9 +107,7 @@ The editor process has access to certain environment variables.
 \fBAGENDA_DIR\fP
 .RS 4
 The home of all agenda note files read and written by agenda. If this is empty
-agenda will attempt to find the \fIxdg-user-dir DOCUMENTS\fP directory if
-\fBxdg-user-dir(1)\fP is found in \fBPATH\fP. Otherwise `$HOME/.agenda` will be
-used.
+`${XDG_DATA_HOME:-"$HOME/.local/share"}/agenda` will be used.
 .PP
 .RE
 \#
@@ -192,11 +190,6 @@ The last agenda file. Can be empty if no previous agenda note exists.
 .RE
 \#=============================================================================
 .SH "SEE ALSO"
-\fBxdg-user-dir(1)\fP
-.RS 4
-Used to find the default \fBAGENDA_DIR\fP.
-.PP
-.RE
 \fBdate(1)\fP
 .RS 4
 Your installation of date determines which date formats for the \fI<date>\fP