Document fct_lump() in its own help topic and clarify superseded status

[mahjabinoyshi]

This PR gives fct_lump() its own help topic and clarifies that it is
superseded in favour of the more explicit helpers.

Changes:

• Add a dedicated roxygen block for fct_lump() describing it as a
  superseded helper that delegates to fct_lump_min(), fct_lump_prop(),
  fct_lump_n(), and fct_lump_lowfreq().
• Keep the existing family documentation for the helper functions and make
  their behaviour clearer.

devtools::document() currently reports some roxygen cross-reference
warnings for links between the helpers and for the %in% operator; I am
happy to adjust these based on your preferred linking style.

Fixes #355.

[hadley]

It looks like you might have accidentally deleted the definition of the other fct_lump functions.

[mahjabinoyshi]

@hadley Thanks for spotting that! I’ve restored all the fct_lump_*() function definitions and given fct_lump() its own superseded help topic, leaving the helpers documented together.Kindly recheck and let me know.

[hadley]

Thank you!