Fix #11050: clarify FloatingPointControl invalidException docs

[AayushMainali-Github]

Rationale

FloatingPointControl documentation incorrectly claimed that enabling invalidException / severeExceptions traps on "uninitialized" floating-point variables.

That is inaccurate: an uninitialized float may hold a valid bit pattern, and default-initialized floating-point values in D are quiet NaNs, which do not by themselves raise the invalid-operation exception.

This updates the module docs and example to describe actual invalid operations (e.g. 0.0/0.0) and to state that quiet NaNs / default-initialized floats do not trigger that trap on their own.

Feature request or issue tracking

Closes #11050.

Pre-review checklist

• I have performed a self-review of my code.
• If my PR fixes a bug or introduces a new feature, I have added thorough tests.
• If my changes are non-trivial and do not concern a reported issue, I have added a changelog entry.

LLM/AI disclosure

This PR was written with the assistance of an LLM/AI.

[ibuclaw]

To fair, it was accurate 10 years ago.

dlang/dmd#9897