doc: fix default limit of maxHeadersCount

The documented limit of 2000 refers to header pairs (counting both
keys and values), which gives an effective limit of 1000 headers.

This applies to both client and server; they have the same mechanism.

Signed-off-by: Aryn H <arynh@protonmail.com>
PR-URL: https://github.com/nodejs/node/pull/65472
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Tim Perry <pimterry@gmail.com>
Reviewed-By: Deokjin Kim <deokjin81.kim@gmail.com>