Document "use isms <Perl5>"

doc/Language/pragmas.pod6

@@ -78,6 +78,28 @@ C<fatal> pragma is enabled by default, and you can I<disable> it with C<no fatal
 
 =item B<invocant> [NYI]
 
+=item X<B<isms>|isms> [2018.06 and later]
+
+Allow for some other language constructs that were deemed to be a trap that
+warranted a warning and/or an error in normal Perl 6 programming.  Currently,
+only C<Perl5> is allowed, which disables a bunch of P5isms warnings and errors.
+
+    sub abs() { say "foo" }
+    abs;
+    # Unsupported use of bare "abs"; in Perl 6 please use .abs if you meant
+    # to call it as a method on $_, or use an explicit invocant or argument,
+    # or use &abs to refer to the function as a noun
+
+In this case, providing an C<abs> sub that doesn't take any arguments, did
+not make the compilation error go away.
+
+    use isms <Perl5>;
+    sub abs() { say "foo" }
+    abs;   # foo
+
+By specifying C<use isms>, the compiler will allow the offending Perl 5
+construct, allowing the code to actually be executed.
+
 =item X<B<lib>|lib> L<Finding Modules|/language/modules#Finding_Modules>
 
 =item X<B<newline>|newline>