Add a note about performance considerations of `stripSpaces`

See https://esbench.com/bench/613f1ca06c89f600a570114b for some related
benchmarks.

src/annotator/anchoring/pdf.js

@@ -315,6 +315,10 @@ async function anchorByPosition(pageIndex, start, end) {
 /**
  * Return a string with spaces stripped and offsets into the input string.
  *
+ * This function optimizes for performance of stripping the main space chars
+ * that PDF.js generates over handling all kinds of whitespace that could
+ * occur in a string.
+ *
  * @param {string} str
  * @return {[string, number[]]}
  */