Move info about Since column to where it belongs

The data in the table is actually not accurate since some of these
symbols aren't part of 6.c language and only will become part of 6.d

Author: zoffixznet

doc/Language/unicode_texas.pod6

@@ -5,9 +5,7 @@
 =SUBTITLE Unicode symbols and their Texas (ASCII) counterparts
 
 The following unicode symbols can be used in Perl 6 without needing to
-load any additional modules.  Please note that the Since column
-applies to the version of Perl the symbol was known to be available.
-It is only mentioned if it is different from C<6.c>.
+load any additional modules.
 
 Reference is made below to various properties of unicode codepoints.
 The definitive list can be found here:
@@ -47,6 +45,8 @@ line), or C<Zp> (Separator, paragraph) property.
 
 This list contains the single codepoints [and their Texas (ASCII)
 equivalents] that have a special meaning in Perl 6.
+The C<Since> column indicates the version of Perl 6 in which 
+the symbol was introduced.
 
 =table
   Symbol | Codepoint | Texas      | Since | Remarks