docs: add ordering note to shift.Rd, character example to nafill.Rd

[LeonidasZhak]

Changes

shift.Rd — ordering note for time-series/panel users

Added a paragraph in the Details section explaining that shift operates on row position, not time order. This is a critical gotcha for users migrating from Stata, where L.var (after xtset) automatically respects time order within panels.

Added an explicit example showing the WRONG (unsorted) vs RIGHT (sorted) approach:

DT = data.table(year=c(2012, 2010, 2011), v1=c(30, 10, 20))
DT[, lag_wrong := shift(v1, 1L)]           # wrong: lag by row position
DT[order(year), lag_right := shift(v1, 1L)] # right: sort first

nafill.Rd — character example

Added a character vector example demonstrating locf and const fill types. Character was listed as a supported type in the Details section but had no example (only numeric and factor examples existed).

Validation

• tools::checkRd() passes on both modified Rd files
• Examples run correctly and produce expected output
• No other open PRs or issues on this topic

Stata connection

The shift ordering note directly addresses the most common Stata-to-R migration pitfall with lag/lead operations: Stata's xtset + L.var handles panel ordering automatically, while data.table's shift requires explicit DT[order(timevar), ...]