docs: threat-model: clarify "security bug" vs "vulnerability"

docs: threat-model: clarify "security bug" vs "vulnerability"

Throwing an LLM (Opus 5) at a file looking for random bugs after having
read the threat model made it ignore certain bugs it found because "the
threat model said they must be ignored". When asked why, the LLM
rightfully reported the ambiguous wording used at a few places:
"is not a security bug", which can also be read as "is not a bug",
despite the rest of the document. That is particularly true when the
LLM decides to look for info using grep.

This change replaces "security bug" with "vulnerability" at a few places
in order to remove this identified ambiguity, and uses "bugs" instead of
issues in two such sentences to insist that what is described remains a
bug.

Cc: Greg KH <greg@kroah.com>
Signed-off-by: Willy Tarreau <w@1wt.eu>
Link: https://patch.msgid.link/20260802203540.3453-2-w@1wt.eu
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>