doc: fix imprecision in IO.FS.Stream.getLine and IO.FS.Handle.getLine

[ia0]

This PR fixes imprecise documentation for IO.FS.Stream.getLine and IO.FS.Handle.getLine. If the stream does not end with a newline (or line break), then getLine will return a String that does not end with a newline. Reciprocally, this situation can only occur when reaching an EOF.

Note that the documentation of read does not necessarily need to be similarly updated. This is because read will always emit an empty array, even after a partially filled array.

This is based on testing only, not necessarily intent of the code. Here are reproduction steps:

def main : IO Unit := do
  repeat do
    let line ← (← IO.getStdin).getLine
    IO.println s!"XXX{line}YYY"

Running with the following input where % indicates EOF (e.g. using Ctrl-D if there was no input since last newline or EOF, or using Ctrl-D twice there was: once to flush the buffer and once to indicate EOF):

hello

foo%%

gives the following output (when YYY does not start a line, it means getLine did not end with a newline):

XXXhello
YYY
XXX
YYY
XXXfooYYY
XXXYYY

[mathlib-lean-pr-testing]

Mathlib CI status (docs):

• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase 8e0b5589d6ad6e4181bc4a98b7dfb17c6c305ba2 --onto 3fc29d37a70f8fd904ebab848557c12383543008. You can force Mathlib CI using the force-mathlib-ci label. (2026-08-11 13:44:45)

[leanprover-bot]

Reference manual CI status:

• ❗ Reference manual CI will not be attempted unless your PR branches off the nightly-with-manual branch. Try git rebase 8e0b5589d6ad6e4181bc4a98b7dfb17c6c305ba2 --onto 3fc29d37a70f8fd904ebab848557c12383543008. You can force reference manual CI using the force-manual-ci label. (2026-08-11 13:44:47)