Remove the duplicate documentation of pem_password_cb(3).

While here, also:

* Avoid the misleading term "default password callback" because none of
the functions in SSL_CTX_use_certificate(3) support overriding it.
* Do not talk about "storing", "writing", and "encryption" since the cb
passed to SSL_CTX_set_default_passwd_cb(3) is never used for any of that.
* List the functions using cb.
* Document what happens by default.
* Remove the misleading words "which must be provided by the application"
because all this is actually optional.
* Make several wordings more precise.
* Below EXAMPLES, fix argument naming to agree with pem_password_cb(3),
clarify the description of what the example does, and, as suggested by tb@,
use strlcpy(3).

OK tb@

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