DOC: add examples to docstrings

[jorisvandenbossche]

This often used functions lacks some illustrating examples

• sort_values ([Issue: #16416] Adding examples for some DataFrame methods #16437)
• reset_values (DOC: Reset index examples #16967)
• set_index (DOC: add examples to set_index method #16467)
• fillna ([Issue: #16416] Adding examples for some DataFrame methods #16437)
• drop ([Issue: #16416] Adding examples for some DataFrame methods #16437)
• pop (DOC: Improving docstring of pop method (#16416) #16520)

[jorisvandenbossche]

Other docstrings that are in the top most viewed and lacking examples: reset_index, set_index, fillna, drop

[geoninja]

@jorisvandenbossche I'd like to take this one.

[VincentLa]

@geoninja any chance you're at Pycon doing sprints right now?

[VincentLa]

I can add some examples for fillna and drop.

[geoninja]

@jorisvandenbossche Both Vincent and I are at PyCon so we will split the work on this one

[geoninja]

I'm going to work on reset_values so we can close this issue.

[jreback]

if anyone at sprints wants to add to .reset_index() we can close this one

@TomAugspurger