R CMD check warning: argument `B` in `getcoeffs()` not documented

[RobinHankin]

* checking for code/documentation mismatches ... OK
* checking Rd \usage sections ... WARNING
Undocumented arguments in documentation object 'Extract.clifford'
  ‘B’

Functions with \usage entries need to have the appropriate \alias
entries, and all their arguments documented.
The \usage entries must correspond to syntactically valid R code.
See chapter ‘Writing R documentation files’ in the ‘Writing R
Extensions’ manual.
* checking Rd contents ... OK