docs: iostats: Rewrite intro, remove outdated formats

The introduction discussed stat file formats for very old kernel versions,
which obscured key information that readers may find useful. Additionally,
the example file contents and the reference to "15 fields" did not account
for the flush fields added in b6866318657 ("block: add iostat counters for
flush requests") [1].

Rewrite the introduction to focus only on the current kernel's disk I/O stat
file formats. Also, clean up wording for conciseness.

Link: https://lore.kernel.org/lkml/157433282607.7928.5202409984272248322.stgit@buzz/T/ [1]

Signed-off-by: David Reaver <me@davidreaver.com>
Link: https://lore.kernel.org/r/20250215180114.157948-1-me@davidreaver.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

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