docs: fix shift.Rd Value section to reflect vector return for single n

[LeonidasZhak]

Changes

Fixed the Value section of shift.Rd to accurately describe the return type behavior.

Problem

The Value section stated:

A list containing the lead/lag of input x.

But shift() returns a vector (not a list) when x is atomic and length(n) == 1. This is confirmed by src/shift.c:181:

if (isVectorAtomic(obj) && length(ans) == 1) ans = VECTOR_ELT(ans, 0);

The Details section already documented this behavior correctly, but the Value section was inaccurate.

Fix

Updated the Value section to:

When x is a vector and length(n) == 1, a vector of the same type and length as x. Otherwise, a list of vectors (one element for each combination of input column and n value).

Stata migration relevance

This fix is particularly important for Stata migrants who expect consistent return types from lag/lead operations. Stata's L.var always returns a vector, and the inaccurate Value section could lead users to believe shift() always returns a list, causing unexpected behavior in their code.

Validation

• tools::checkRd('man/shift.Rd') passes (no errors)
• Change is purely documentation (no code changes)
• No other open PRs address this specific issue

Duplicate check

• PR docs: add ordering note to shift.Rd, character example to nafill.Rd #7774 adds ordering notes and panel data examples but does NOT fix the Value section
• No other PRs address the Value section inaccuracy

[LeonidasZhak]

I am withdrawing this PR as part of a broader cleanup of an oversized automated contribution batch. I am sorry for the review noise and the pressure this may have put on maintainers. Going forward I will keep contributions to this project much more limited and higher-signal, ideally only one or two focused PRs at a time. Thank you for maintaining the project.

[ben-schwen]

@LeonidasZhak please read our contribution guidelines regarding the use of AI