Convert to HTML4 named entity (&) escapes
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We support several output formats:
* html (supports all Unicode characters)
* man (supports all Unicode characters)
* pdf (supports only Latin-1 characters)
* info
While some output formatting tools support all Unicode characters,
others only support Latin-1 characters. Specifically, the PDF rendering
engine can only display Latin-1 characters; non-Latin-1 Unicode
characters are displayed as "###".
Therefore, in the SGML files, we can only use Latin-1 characters. We
can use UTF8 representations of Latin-1 characters, or HTML entities of
Latin-1 characters, e.g., Álvaro.
Do not use UTF numeric character escapes (&#nnn;).
When building the PDF docs, problem characters will appear as warnings.
