Add Warning About Handling Private Keys To Transaction Examples

[harding]

(As an embarrassing footnote, the only time I ever handled private keys on mainnet, I lost $4.00 worth of bitcoins. Lesson learned.)

[saivann]

I also thought this code example could be sensitive if used directly, good idea! Maybe we can shorten it a little and also display it in the P2SH example, since it is also using dumprivkey?

**Warning:** Users should never manually manage private keys on mainnet.
Making a mistake with a private key can be very costly---as in the case of a HD
wallet [cross-generational key compromise][devguide hardened keys].
These examples are to help you learn, not for you to emulate on
mainnet.

[harding]

@saivann I liked how the previous phrasing re-enforced the idea that raw txes are unsafe.

I agree that we should add another warning to the P2SH section. How about we keep the original warning in the Complex section and add a shorter warning with a link in the P2SH section? E.g.

Reminder: Users should never manually manage private keys on mainnet. See the warning in the [complex raw transaction section][devex complex raw transaction].

[saivann]

@harding LGTM

[harding]

@saivann done in commit 85f2709. Thanks! (If it LGT you, I think its good to merge. I tested the new links.)

[saivann]

@harding Thanks!