docs: note the intentional density + verbose-comments tradeoff

Add a 5-line style note at the top of lua/tiny-jump/init.lua and
mirror the same message in the README under the size-comparison
section. The comments in this file are load-bearing — they do the
explanatory work that would otherwise be done by helper functions
and verbose names. Prevents future-me (or a drive-by PR) from
'cleaning up' the comments as redundant.

lua/tiny-jump/init.lua

@@ -5,6 +5,11 @@
 -- This Source Code Form is subject to the terms of the Mozilla Public
 -- License, v. 2.0. If a copy of the MPL was not distributed with this
 -- file, You can obtain one at http://mozilla.org/MPL/2.0/.
+--
+-- Style note: this file is intentionally dense (126 SLOC in one file).
+-- Comments are thorough by design — they do the explanatory work normally
+-- done by helper functions and verbose names. If you're editing here, read
+-- the comments first.
 
 local api, fn, M = vim.api, vim.fn, {}