Details from man-pages(7) that are used:
Formatting conventions for manual pages describing functions
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Variable names should, like argument names, be specified in italics.
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Formatting conventions (general)
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Special macros, which are usually in uppercase, are in bold.
Exception: don't boldface NULL.
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Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/10042)