Dated documentation for PairList structure

[mcsloy]

The documentation provided for the PairList structure is outdated. It makes references to fields that no longer exist, such as r, and fails to mention others, such as s.

NeighbourLists.jl/src/types.jl

Lines 10 to 48 in e930bd6

	"""
	`PairList` stores a neighbourlist as a list of pairs
	```
	PairList(X, cutoff, cell, pbc)
	PairList(nlist::CellList)
	```
	where
	* `X` : positions, either as 3 x N matrix or as `Vector{SVec}`
	* `cutoff` : positive real value
	* `cell` : 3 x 3 matrix, with rows denoting the cell vectors
	* `pbc` : 3-tuple or array, storing periodicity information
	### Kw-args:
	* `int_type` : default is `Int`
	* `store_first` : whether to store the array of first indices, default `true`
	* `sorted` : whether to sort the `j` vector, default `false`
	### PairList fields
	`X, cutoff, i, j, r, R, first`, where
	`(i[n], j[n])` denotes the indices of a neighbour pair, `r[n]` the distance
	between those atoms, `R[n]` the vectorial distance, note this is identical to
	`X[i[n]]-X[j[n]]` without periodic b.c.s, but with periodic boundary conditions
	it is different. `first[m]` contains the index to the first `(i[n], j[n])` for
	which `i[n] == first[m]`, i.e., `(j, first)` essentially defines a compressed
	column storage of the adjacancy matrix.
	"""
	struct PairList{T <: Real, TI <: Integer}
	X::Vector{SVec{T}}
	C::SMat{T}
	cutoff::T
	i::Vector{TI}
	j::Vector{TI}
	S::Vector{SVec{TI}}
	first::Vector{TI}
	end