update docs for lacosmic sepmed parameter [docs only]

[jradavenport]

Please have a look at the following list and replace the "[ ]" with a "[x]" if
the answer to this question is yes.

• For new contributors: Did you add yourself to the "Authors.rst" file?

For documentation changes:

• For documentation changes: Does your commit message include a "[skip ci]"?
  Note that it should not if you changed any examples!

  My commit message used: [docs only], which I assume also behaves like [skip ci]

For bugfixes:

• Did you add an entry to the "Changes.rst" file?
• Did you add a regression test?
• Does the commit message include a "Fixes #issue_number" (replace "issue_number").
• Does this PR add, rename, move or remove any existing functions or parameters?

For new functionality:

• Did you add an entry to the "Changes.rst" file?
• Did you include a meaningful docstring with Parameters, Returns and Examples?
• Does the commit message include a "Fixes #issue_number" (replace "issue_number").
• Did you include tests for the new functionality?
• Does this PR add, rename, move or remove any existing functions or parameters?

Please note that the last point is not a requirement. It is meant as a check if
the pull request potentially breaks backwards-compatibility.

---

Fixes #631 by clarifying the sepmed parameter is for performance only, and not part of the L.A. Cosmic algorithm natively.

Also corrected typo of "lacosmic" to "L.A. Cosmic", per the original paper.

[bsipocz]

no need for changelog for docs only changes.

[bsipocz]

Welcome @jradavenport!

[bsipocz]

My commit message used: [docs only], which I assume also behaves like [skip ci]

Indeed, it's similar. It's a "home grown" option that only works with travis, so here it works as intended.

[MSeifert04]

@jradavenport Thank you for the PR 👍

Could you remove the changelog entry - Then I think this can be merged.

[mwcraig]

Thanks @jradavenpor, merging!