Expand descriptions of LSH-relevant arguments in README and help messages

[haydenm]

This PR expands descriptions of arguments that use locality-sensitive hashing—to lower runtime and memory usage on especially large inputs—in the README and help messages. Namely, the PR:

• Adds technical detail to the arguments in the README, borrowing from the help messages
• Adds detail on why certain values for these arguments are recommended and reasonable in practice
• Makes minor changes to the help messages

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (895a7e3) 93.73% compared to head (d178bd4) 93.73%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #57 +/- ## ======================================= Coverage 93.73% 93.73% ======================================= Files 63 63 Lines 6115 6115 ======================================= Hits 5732 5732 Misses 383 383 ``` | [Flag](https://app.codecov.io/gh/broadinstitute/catch/pull/57/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=broadinstitute) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/broadinstitute/catch/pull/57/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=broadinstitute) | `93.73% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=broadinstitute#carryforward-flags-in-the-pull-request-comment) to find out more.

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