SQL: Doc and test adjustments

FU/SQL.pm

@@ -180,8 +180,7 @@ B<String literals> are interpreted as raw SQL fragments.
 
 =item 2.
 
-Objects returned by other functions listed below can be included as
-SQL fragments.
+Objects returned by other functions listed below are included as SQL fragments.
 
 =item 3.
 
@@ -200,8 +199,8 @@ These rules allow for flexible SQL construction:
 There is some magic going on in order to differentiate between a I<string
 literal> and other arguments. The rule is that anything that is
 C<builtin::created_as_string()> and read-only (as per
-C<Internals::SvREADONLY()>) is considered raw SQL. Variables are by definition
-writable:
+C<Internals::SvREADONLY()>) is considered raw SQL. Regular variables, array
+elements and hash values are always writable:
 
   my $x = 'SELECT 1';
   SQL $x;  # BAD: $x is used as bind parameter instead
@@ -210,8 +209,16 @@ writable:
   my $x = SQL 'SELECT 1';
   SQL $x;
 
-In most cases, though, this is the behavior you want. In the few cases where it
-isn't, you can always use C<P()> or C<RAW()> to force an argument as bind
+Constants created with C<use constant> are considered string literals, and
+there are probably plenty of other creative ways to end up with variables that
+may be considered a "string literal" by this module. L<Hash::Util>,
+L<Scalar::Readonly>, other modules and tied hashes or arrays all have the
+potential to create read-only strings, but I don't expect that these are
+commonly applied on untrusted user input.
+
+In most cases, this heuristic should work out well. In the few cases where it
+doesn't, or when you're not entirely sure what kind of value you're dealing
+with, you can always use C<P()> or C<RAW()> to force an argument as bind
 parameter or SQL string.
 
 =item P($val)
@@ -235,7 +242,7 @@ Force the given C<$sql> string to be included as SQL. For example:
   SQL 'WHERE * FROM', RAW $tables[1];
   # 'SELECT * FROM b'
 
-Absolutely do not use this function with untrusted input.
+Never use this function with untrusted input.
 
 =item PARENS(@args)
 
@@ -271,7 +278,7 @@ C<'1=1'> (i.e. true) if C<@conditions> is an empty list.
 
 =item AND($hashref)
 
-A special form of C<AND()> that tests the given column for equality instead.
+A special form of C<AND()> that tests the given columns for equality instead.
 The keys of the hashref are interpreted as raw SQL and the values as bind
 parameters.
 
@@ -335,8 +342,7 @@ convenient insert-or-update:
   SQL 'INSERT INTO table', VALUES($data),
       'ON CONFLICT (name) DO UPDATE', SET($data);
 
-(The bind parameters are duplicated though, there's no duplicate detection yet.
-Not sure if that's even worth it)
+(The bind parameters are duplicated though)
 
 =item IN($arrayref)