Make corrections to docs

[ColeFrench]

Hi! This is my first time contributing to this project, and I hope to make (many) more contributions.

This PR generally "updates" the Tectonic book. The substance of these updates is ensuring the documented command usage matches the actual usage displayed by the CLI. This fixes #1057 and other issues people may open about this.

To review those corrections, you can look at these commit ranges:

• V1 CLI: https://github.com/tectonic-typesetting/tectonic/pull/1065/files/4421ba054c915c716b0232eff925e97c1de70178..f6ec72922d49f0b7d55fafcee2071504d409a3ef
• V2 CLI: https://github.com/tectonic-typesetting/tectonic/pull/1065/files/38724e8ef9e2ca616957b55b961cdc6112b5269c..2f7f2f3348f5c4d52fa88964166af2b135497941

Since I am doing some bulk text changing and formatting anyway, I thought I would include grammar fixes in this PR as well. Those make up the rest of the file changes.

[codecov]

Codecov Report

Merging #1065 (86f3312) into master (a2865a6) will increase coverage by 0.02%.
The diff coverage is n/a.

❗ Current head 86f3312 differs from pull request most recent head 96d77dc. Consider uploading reports for the commit 96d77dc to get more accurate results

@@            Coverage Diff             @@
##           master    #1065      +/-   ##
==========================================
+ Coverage   45.83%   45.85%   +0.02%     
==========================================
  Files         171      171              
  Lines       64145    64180      +35     
==========================================
+ Hits        29398    29427      +29     
- Misses      34747    34753       +6     

Impacted Files	Coverage Δ
src/unstable_opts.rs	57.37% <ø> (+0.71%)	⬆️

... and 29 files with indirect coverage changes

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[pkgw]

Wow, thank you! I appreciate the careful editing and polish that you've put in here. The changes aren't always obvious but I think that it's quite valuable to correct things like "which"/"that" mistakes, and clearly you've given things a close read.

If you're motivated, any further contributions to the documentation are extremely welcome — unfortunately, Tectonic has heretofore continued the long-standing TeX tradition of having rather poor HTML documentation, which is something that I would love to see change. I have my own projects aiming to tackle that, but I've completely overwhelmed by work responsibilities lately (I feel like I've been writing that for the past 3-4 years ...) and it would be fantastic to have more people working on things!

[ColeFrench]

Thanks! I'm glad my contributions are welcome. Hopefully, as I get more in tune with the code, I'll find more things to improve or add to the documentation.