add explanations for some strings that match BASIC rom

strings/0249FF

@@ -0,0 +1,3 @@
+fragment/ EOR #$FF
+
+Single instruction. Not copyrightable.

strings/454741

@@ -0,0 +1,3 @@
+word fragment "EGA"
+
+A fragment of a word is not copyrightable

strings/600000

@@ -0,0 +1,3 @@
+RTS followed by blank space
+
+Single instruction. Not copyrightable.

strings/68A868

@@ -0,0 +1,4 @@
+PLA / TAY / PLA
+
+Pull Y and A from stack.
+Not copyrightable.