Doc: Use ++n instead of +p in charmrun examples

[hizv]

Resolves #2947.

• Keeps +p in some examples (e.g. ./hello) to show that it's fine to use +p for testing.
• Adds an example to illustrate how +p relates to ++n in the SMP options section.

[hizv]

Should AMPI examples retain +p? The introduction section of AMPI uses P=4 in a diagram, so it might be easier to relate the +p option to that.