Revise Finding the Source chapter

[hturner]

This chapter needs revising now that GitHub search includes search navigation for R.

For instance, we can search repo:r-devel/r-svn symbol:compcases to find the header and C files corresponding to symbol "compcases" - this produces more relevant results than [compcases path:*.c](repo:r-devel/r-svn compcases path:*.c).

As another example, we can search repo:r-devel/r-svn symbol:quantile and this returns the R files containing the quantile S3 generic and corresponding methods (quantile.default, quantile.POSIXt, and quantile.ecdf`).

So this simplifies lookup on GtiHub considerably.