trim-alignment : documentation updates

[nbokulich]

the parameter description should explicitly state that primers should be in 5'-3' orientation and should not be reverse complemented. A little more detail in the description section could also help clarify usage.

Also, there are some typos/missing spaces in the help docs that could be cleaned up at the same time.

[colinbrislawn]

I took a look into this. As far as I can see, this command is not mentioned in the main rescript tutorial or the bonus extract-reads tutorial.

Given that these two commands are superficially similar, a tutorial to show intended usage of this function and contrast with extract-reads may be helpful.

[mikerobeson]

Just wanted to leave a note here that a draft tutorial for trim-alignment is being compiled in the QIIME 2 forum. This tutorial is not public yet.