Fix documentation errors in Folly when building with Xcode 12

Summary:
Fix the documentation errors in Folly when building with Xcode 12. Xcode supports validating doxygen comments by enabling the `-Wdocumentation` flag. In Clang 12, a new feature was added to warn when an inline Doxygen comment has no argument (https://reviews.llvm.org/rL367809).

`\a` and `\b` are special commands in doxygen and were being flagged as errors. I enclosed the example escape sequences within a code block to fix these errors. You can read about all the special doxygen commands here: https://www.doxygen.nl/manual/commands.html

Reviewed By: yfeldblum

Differential Revision: D22488410

fbshipit-source-id: bc4acf31b3df2a860202d0bd1ee356ce8f8fe49f

folly/String.h

@@ -70,9 +70,11 @@ String cEscape(StringPiece str) {
  *
  * Recognizes the standard C escape sequences:
  *
+ * \code
  * \' \" \? \\ \a \b \f \n \r \t \v
  * \[0-7]+
  * \x[0-9a-fA-F]+
+ * \endcode
  *
  * In strict mode (default), throws std::invalid_argument if it encounters
  * an unrecognized escape sequence.  In non-strict mode, it leaves