Editing P.9: "Don't waste time or space" Example Text (#1439)

* Initial rewrite

* Fixed a couple of inaccuracies and minor grammar mistakes

Thanks to twitter user @lunasorcery for these changes!

* Added toLower to dictionary

CppCoreGuidelines.md

@@ -1055,8 +1055,8 @@ There are several more performance bugs and gratuitous complication.
         for (int i = 0; i < strlen(s); ++i) s[i] = tolower(s[i]);
     }
 
-Yes, this is an example from production code.
+This is actually an example from production code.
-We leave it to the reader to figure out what's wasted.
+We can see that in our condition we have `i < strlen(s)`. This expression will be evaluated on every iteration of the loop, which means that `strlen` must walk through string every loop to discover its length. While the string contents are changing, it's assumed that `toLower` will not affect the length of the string, so it's better to cache the length outside the loop and not incur that cost each iteration.
 
 ##### Note
 

scripts/hunspell/isocpp.dic

@@ -564,6 +564,7 @@ tmp
 TMP
 tock
 TODO
+toLower
 toolchains
 TotallyOrdered
 TP