document Xresources application namespace explicitly (#488)

A decent amount of users have been confused by this.
So make it more explicit to avoid confusion.

Co-authored-by: NRK <nrk@disroot.org>
Reviewed-on: https://codeberg.org/nsxiv/nsxiv/pulls/488
Reviewed-by: NRK <nrk@disroot.org>
Co-authored-by: fxdn <fxdn@noreply.codeberg.org>
Co-committed-by: fxdn <fxdn@noreply.codeberg.org>

etc/nsxiv.1

@@ -440,7 +440,9 @@ Zoom in.
 .B Button5
 Zoom out.
 .SH CONFIGURATION
-The following X resources are supported:
+The following X resources are supported under "Nsxiv" (e.g.
+.B Nsxiv.bar.font
+):
 .TP
 .B window.background
 Color of the window background