Docs: LogAccessor docstrings

[seeM]

Add docstrings to each LogAccessor method. Each should contain a full, simple, working example using doctest. We could re-use the accessor tests.

Checklist:

• dropna
• drop_duplicates
• query
• head
• tail
• sample
• drop
• set_index
• reset_index
• rename
• pivot
• melt
• fillna
• bfill
• ffill

These can eventually be used to auto-generate reference docs (#1).