Update pair-list buffer manual
Several corrections and updates to the manual section that explains
the pair-list buffer estimation:
* Clarified the role of i and j particles in the equations
* Added missing factors t and t^2 for several sigmas
* Added missing term in expansion in intermediate result
* Added text explaining the approximations made
* Rewrote the paragraph on how constrained particles are handled,
which matched the code in 4.6, not 5.1 (or 5.0)
Note that the code and code documentation are correct and up to date.
Change-Id: Iaf707f1c5faeebd6b9b099753735a1c61b903f9c