clarify restrictions vs bad practice

Author: tbrowder

doc/Language/tables.pod6

@@ -32,7 +32,9 @@ illegal.
 3. The only valid row separator characters are V<'_'>, V<'-'>, V<'+'>, V<' '>,
 V<'|'>, and V<'='>.
 
-4. Consecutive row separator lines are illegal (but only generate a warning).
+4. Consecutive interior row-separator lines are illegal.
+
+5. Leading and trailing row-separator lines generate a warning.
 
 =head1 Best Practices
 
@@ -43,7 +45,9 @@ the problem.
 2. Align table columns and rows carefully.  See the examples in later
 best practices.
 
-3. For tables with a heading and single- or multi-line content, use
+3. Don't use visual borders on the table.
+
+4. For tables with a heading and single- or multi-line content, use
 one or more contiguous equal signs (V<'='>) as the row separator after
 the heading, and use one or more contiguous hyphens (V<'-'>) as the row
 separator in the content portion of the table.  For example,
@@ -226,7 +230,7 @@ r1c0    r0c1   r0c3
 =end table
 =end code
 
-=item Two consecutive row separators are not allowed:
+=item Two consecutive interior row separators are not allowed:
 
 =begin code :skip-test
 =begin table