More information on vignettes regarding how searches are performed

**Is your feature request related to a problem? Please describe.**
I use the package but i dont know how it performs searches on specific arguments and these should be further documented and described in the vignette.

**Describe the solution you'd like**
A clear and concise description of what and how the arguments do searches for specific arguments

**Describe alternatives you've considered**
Asking the maintainer which is not efficient

**Additional context**
The arguments that need more information regarding the searches are as follows
getCandidateCodes() - how is the search on the keywords done? exact match / fuzzy?
summariseOrphanCodes() - again is this based on fuzzy matching? what tables is it looking at?
summariseUnmappedCodes() - how is this done and where does it look for this information? from the source_concept_id? doesnt it need to be in the concept table?