FIPS readme: streamline FIPS preference eval

[dagood]

• Some updates to the doc change in Address GOFIPS short-circuit bug #2244

Streamline the FIPS mode preference eval list to (try to) make it easier to take in. Expand on why someone would potentially want Disabled ❌. Clarify(-ish) that the fix isn't in yet by specifying (As of Go 1.27, 1.26.3-1, and 1.25.10-1.).

[dagood]

I considered a mermaid diagram for the sequence:

flowchart LR
    A[Program starts] --> B{GODEBUG fips140\nsetting present?}
    B -- Yes --> B2{fips140 value?}
    B2 -- "off" --> Disabled["Disabled ❌"]
    B2 -- "on | only | debug" --> Enabled["Enabled ✅"]
    B2 -- "[other]" --> C
    B -- No --> C{GOFIPS=1 or\nGOLANG_FIPS=1?}
    C -- Yes --> Enabled
    C -- No --> D{Platform-specific\npreference detected?}
    D -- Yes --> Enabled
    D -- No --> E{requirefips\nbuild option?}
    E -- Yes --> Enabled
    E -- No --> NoPref["No preference ❔"]

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...But ultimately, I think it's simple enough already.

[Copilot]

Pull request overview

Updates the FIPS documentation to make the runtime “FIPS preference evaluation” logic easier to follow and to better explain the rationale for explicitly disabling FIPS preference.

Changes:

• Rewrites the runtime preference-evaluation list into a shorter, ordered set of rules.
• Adds a “Disabled ❌” behavior section and an explanatory note (container scenario) for why disabling may be desirable.
• Adds a version-scoping note indicating when GODEBUG=fips140=off behavior applies.