More draft updates and additions.

This patch
 * expands sections on LPC and LTP synthesis into something that can
    actually be implemented
 * fixes an error in the excitation reconstruction
 * reverts an erroneous simplification of the subframe gain
    decoding, and
 * updates the LPC gain limiting description to reflect the new,
    more accurate approach for computing the reflection
    coefficients.
It also includes a number of general clean-ups, such as
 * correcting the description of the sample rates various pieces
    run at (e.g., we can decode directly to rates other than 48 kHz)
 * the usage of "sampling rate" vs. "sample rate"
 * capitalization consistency in TOC titles, and
 * better selection of which sections appear in the TOC.