coherence() documentation

Ran into an issue with fitting coherence() into my workflow with external term co-occurrence matrices, figured it out now but letting you know that the documentation could reflect this:

> The term co-occurrence matrix, e.g, a Matrix::sparseMatrix or base::matrix, serving as the reference to calculate coherence metrics. Please note that a memory efficient version of the tcm is assumed as input with all entries in the lower triangle (excluding diagonal) set to zero (see, e.g., create_tcm).

coherence() actually works properly with Matrix class dsCMatrix (which is what is produced in the example workflow), but does not work with class dgCMatrix with empty lower triangle. This is because

> tcm = as.matrix(tcm[top_terms_tcm, top_terms_tcm])

in coherence() doesn't produce the wanted result with lower-triangle empty dgCMatrix. However, a symmetrical dgCMatrix produces the same results as a dsCMatrix.

Given dgCMatrix is the default for converting to sparseMatrix with Matrix, it might be nice to note this in the documentation.