Clarify guidance for use of a BOM as a UTF-8 encoding signature
https://www.unicode.org/L2/L2021/21038-bom-guidance.pdf

・Do not use U+FEFF as a ZWNBSP character; use U+2060 WORD JOINER instead.
・Include a BOM if one is known to be required by a targeted protocol.
・Otherwise, include a BOM when authoring a UTF-8 text file that contains non-ASCII characters,
　is not targeting a specific protocol, and may be opened by applications that will not assume UTF-8 by default
　(this is useful on systems like Microsoft Windows where some applications assume text files to be encoded with the Active Code Page).
・Otherwise, do not include a BOM.