restore the addKey comment, reworded for anchors

Per @dschaper, restore the addKey clarifying comment. It has
been reworded to describe the use of anchors where before it
referenced using grep's  'match only an entire line' argument.

Signed-off-by: William Blew <william@kulian.org>

advanced/Scripts/utils.sh

@@ -57,6 +57,10 @@ addKey(){
   # touch file to prevent grep error if file does not exist yet
   touch "${file}"
 
+  # Match key against entire line, using both anchors. We assume
+  # that the file's keys never have bounding whitespace. Anchors
+  # are necessary to ensure the key is considered absent when it
+  # is a substring of another key present in the file.
   if ! grep -q "^${key}$" "${file}"; then
     # Key does not exist, add it.
     echo "${key}" >> "${file}"