Explain the table of contents script better, and simplify it a bit

test/script/check-toc

@@ -19,7 +19,7 @@ toc_end_line="$(expr "$toc_start_line" + "$toc_section_size" - 2)"
 
 toc_file="$(mktemp -t table-of-contents.XXXXXXXX)"
 heading_file="$(mktemp -t headings.XXXXXXXX)"
-unsorted_toc_file="$(mktemp -t ale.txt.XXXXXXXX)"
+tagged_toc_file="$(mktemp -t ale.txt.XXXXXXXX)"
 sorted_toc_file="$(mktemp -t sorted-ale.txt.XXXXXXXX)"
 
 sed -n "$toc_start_line,$toc_end_line"p doc/ale.txt \
@@ -43,6 +43,8 @@ exit_code=0
 in_section=0
 section_index=0
 
+# Prefix numbers to table of contents entries so that sections aren't mixed up
+# with sub-sections when they are sorted.
 while read -r; do
     if [[ "$REPLY" =~ ^\  ]]; then
         if ! ((in_section)); then
@@ -56,17 +58,15 @@ while read -r; do
         fi
     fi
 
-    echo "$section_index $REPLY" >> "$unsorted_toc_file"
+    echo "$section_index $REPLY" >> "$tagged_toc_file"
 done < "$toc_file"
 
-
-sort -h "$unsorted_toc_file" | sed 's/[0-9]\+//' > "$sorted_toc_file"
-sed -i.bak 's/[0-9]\+//' "$unsorted_toc_file"
-rm -f "$unsorted_toc_file".bak
+# Sort the sections and sub-sections and remove the tags.
+sort -h "$tagged_toc_file" | sed 's/[0-9]\+ //' > "$sorted_toc_file"
 
 echo 'Check for bad ToC sorting:'
 echo
-diff -U2 "$sorted_toc_file" "$unsorted_toc_file" || exit_code=$?
+diff -U2 "$sorted_toc_file" "$toc_file" || exit_code=$?
 
 echo 'Check for mismatched ToC and headings:'
 echo
@@ -74,7 +74,7 @@ diff -U3 "$toc_file" "$heading_file" || exit_code=$?
 
 rm "$toc_file"
 rm "$heading_file"
-rm "$unsorted_toc_file"
+rm "$tagged_toc_file"
 rm "$sorted_toc_file"
 
 exit "$exit_code"