Recommend just .pm6, .p6 and .t

Resolves #778. Extra text removed because I don't see any reason to
explain the raisins and controversy.

doc/Language/modules.pod6

@@ -30,12 +30,8 @@ keyword N<As L<synopsis S11|https://design.perl6.org/S11.html#Units> says:
 Confusing? Yes it is.>, a C<t> directory for tests, and possibly a C<bin>
 directory for executable programs and scripts.
 
-Source files generally use the standard C<.pm> extension, and scripts or
-executables use C<.pl>.  However, if you wish to highlight that the file is
-written in Perl 6 you can use the C<.pm6> extension for modules, and
-C<.p6> or C<.pl6> extension for scripts.  Test files still use the normal C<.t>
-extension. For windows compatibility, C<.p6> is the preferred extension for
-cross-platform scripts.
+Source files generally use the C<.pm6> extension, and scripts or
+executables use the C<.p6>. Test files use the C<.t> extension.
 
 =head2 Loading and Basic Importing