Reworded a few sections, this will need changes if #589 gets merged. Notably:

• The HostFunction section now comes first, so I removed the reference to "using again memory" because this is the first time we are seeing it
• I have also rounded up the section to introduce and clarify the concept of "host", "guest"
• the description of the HostFunction should change to reflect Rename ExternalValues to ImportValues; order of params in ImportFunction,HostFunction. #589
• we should also update this same section to use the Store from the start
• we might describe later the alternate method (passing in "imports"/"external values" directly) for low-level access

EDIT: I did all the last 3 above as #589 was merged in the meantime