doc: Clarify -blocksdir usage

This commit attempts to clarify and correct the `-blocksdir` argument
description and default value. `-blocksdir` does not refer to the full
path to the actual `blocks` directory, but rather the root/parent
directory which contains the `blocks` directory. Accordingly, the
default value is `<datadir>` and not `<datadir>/blocks`. It also
attempts to clarify that only the `.dat` files containing block data are
impacted by `-blocksdir`, not the index files.

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