Align Mu.so & Mu.not docs with design

The previous explanation of so() (double-negation) wasn't consistent
with implementation or mentioned in design docs. It's a common
JavaScript idiom but not relevant to Perl 6. This aligns with the docs
for prefix:<so> and :<not>, too. Ref #2693.

Author: softmoth

doc/Type/Mu.pod6

@@ -609,9 +609,10 @@ Returns the given item to the enclosing C<gather> block, without introducing a n
 
     method so()
 
-Returns a C<Bool> value representing the result of negating (with implicit
-conversion to C<Bool>) the expression and then again computing the negation of
-the result, thus computing the C<Bool> equivalent to the original expression.
+Evaluates the item in boolean context (and thus collapses Junctions),
+and returns the result.
+It is the opposite of C<not>, and equivalent to the
+L«C<?> operator|/language/operators#prefix_?».
 
 One can use this method similarly to the English sentence: "If that is B<so>,
 then do this thing".  For instance,
@@ -622,27 +623,24 @@ then do this thing".  For instance,
         say "Verbose option detected in arguments";
     } # OUTPUT: «Verbose option detected in arguments␤»
 
-The C<$verbose-selected> variable in this case contains a
-L<C<Junction>|/type/Junction>, whose value is C<any(any(False, False),
-any(False, False), any(False, False), any(True, False))>. That is actually a
-I<truish> value; thus, negating it will yield C<False>. The negation of that
-result will be C<True>. C<so> is performing all those operations under the hood.
-
 =head2 method not
 
     method not()
 
-Returns a C<Bool> value representing the logical negation of an expression.
-Thus it is the opposite of C<so>.
+Evaluates the item in boolean context (and thus collapses Junctions),
+and negates the result.
+It is the opposite of C<so>, and equivalent to the
+L«C<!> operator|/language/operators#prefix_!».
 
     my @args = <-a -e -b>;
     my $verbose-selected = any(@args) eq '-v' | '-V';
     if $verbose-selected.not {
         say "Verbose option not present in arguments";
     } # OUTPUT: «Verbose option not present in arguments␤»
 
-Since there is also a prefix version of C<not>, the above code reads better
-like so:
+Since there is also a
+L«prefix version of C<not>|/language/operators#prefix_not»,
+this example reads better as:
 
     my @args = <-a -e -b>;
     my $verbose-selected = any(@args) eq '-v' | '-V';