Index some headings in unicode_entry

Author: AlexDaniel

doc/Language/unicode_entry.pod6

@@ -15,6 +15,7 @@ General information about entering unicode under various operating systems
 and environments can be found on the Wikipedia L<unicode input page|https://en.wikipedia.org/wiki/Unicode_input>.
 
 =head1 XCompose (Linux)
+X<|XCompose>
 
 Xorg includes digraph support using a
 L<I<Compose key>|https://en.wikipedia.org/wiki/Compose_key#GNU.2FLinux> .
@@ -52,6 +53,7 @@ and that should work. Other desktop environments will be different. Look up
 how to set environment variables in yours or use the system-wide option above.
 
 =head2 ibus
+X<|ibus>
 
 If you have problems entering high codepoint symbols such as B<🐧> using the
 C<xim> input module, you can instead use ibus. You will have to install the ibus
@@ -75,6 +77,7 @@ C<--desktop=plasma>. Click OK. It should now launch automatically when you
 log in again.
 
 =head1 WinCompose (Windows)
+X<|WinCompose>
 
 L<WinCompose|https://github.com/samhocevar/wincompose> adds
 L<compose key|https://en.wikipedia.org/wiki/Compose_key> functionality to Windows.
@@ -93,6 +96,7 @@ C<.XCompose> in C<%USERPROFILE%>, or editing user-defined sequences in the optio
 =head1 Editors and shells
 
 =head2 Vim
+X<|Vim>
 
 In L<Vim|https://www.vim.org/>, unicode characters are entered (in
 insert-mode) by pressing first C<Ctrl-V> (also denoted C<^V>), then C<u> and
@@ -125,6 +129,7 @@ equivalents. This will convert the ASCII based ops on the fly while typing
 them.
 
 =head2 Emacs
+X<|Emacs>
 
 In L<Emacs|https://www.gnu.org/software/emacs/>, unicode characters are
 entered by first entering the chord C<C-x 8 RET> at which point the