Mention accuracy of Base.summarysize in docstring

[xlxs4]

From #32881 (comment):

Didn't we at some point explicitly state in the docs that it's a good-effort rough estimate and should not be relied upon? Or perhaps that was just a Jameson comment on GitHub that never got incorporated into the docs. In any case, it seems like we can improve the good-effort to best-effort in some places, and perhaps for the others we should document that it'll never be 100% perfect for the remainder.

While Base.summarysize is being improved upon (e.g., see #54606), it's not 100% accurate. Some example issues:

• should Base.summarysize include alignment? #32881
• Base.summarysize misses null byte of String #54023
• Base.summarysize(::Symbol) returns zero #55549

I saw there's people advocating for a better sizeof docstring (#54007). Do you think adding a note stating that summarysize isn't to be relied upon (and, ideally, in which cases it is accurate) is worth the effort? IMO this isn't clear enough as things currently stand.