[Docs] use self.cap in heap allocated array example

[illiasheshyn]

Just some minor neat-picks spotted as I was reading through Life of a value page. The corresponding issue #4127

Changes

• 3 cases of implicit conversion are now mentioned, replaces both with all
• uses self.cap to allocate memory (consistent with the previous HeapArray example on the same page)

[illiasheshyn]

• added a sneaky change to render the TODO as a markdown note in the Homogeneous variadic arguments

feels wrong to lump unrelated changes under this issue, I'm happy to raise the last commit in a separate issue if needed.

[JoeLoser]

Thanks for the doc edits! No worries about lumping in the minor change in mojo/docs/manual/functions.mdx in this case 👍

[JoeLoser]

!sync