Fix composable function typos. Clarify wording.

[manzaloros]

Summary of changes:

1. Fix small typos and wording for composable functions description.

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[wbamberg]

Thanks! I agree that the current text here is very unclear. I had a couple of suggestions though - in particular I don't think technical writing should ever call something "simple" as a general judgement.

[wbamberg]

I pushed a couple of final changes:

• we use sentence case "Defining composable functions" rather than title case "Defining Composable Functions"
• removed the code comment and integrated the Wikipedia link

Thanks for your work on this PR @manzaloros , it's a real improvement to the docs!

[manzaloros]

Thank you, @wbamberg , I appreciate your compliment and your help.