[openssl/openssl] ee60e1: doc: improve documentation of EVP in-place encryption

[Matthias St. Pierre]

  Branch: refs/heads/openssl-3.1
  Home:   https://github.com/openssl/openssl
  Commit: ee60e1551d34ffae7141657f6504a2201c4bcd20
      https://github.com/openssl/openssl/commit/ee60e1551d34ffae7141657f6504a2201c4bcd20
  Author: Matthias St. Pierre <matthias.st.pierre at ncp-e.com>
  Date:   2023-12-12 (Tue, 12 Dec 2023)

  Changed paths:
    M doc/man3/EVP_EncryptInit.pod
    M doc/man7/provider-cipher.pod

  Log Message:
  -----------
  doc: improve documentation of EVP in-place encryption

The EVP interface explicitly allows in-place encryption/decryption,
but this fact is just 'partially' documented in `EVP_EncryptUpdate(3)`
(pun intended): the manual page mentions only operation failure in
case of 'partial' overlaps. This is not even correct, because
the check for partially overlapping buffers is only implemented
in legacy code paths.

Currently, in-place encryption/decryption is only documented for
RSA (`RSA_public_encrypt(3)`) and DES (`DES_ecb_encrypt(3)`), as
well as in the provider interface (`provider-cipher(7)`).

This commit amends `EVP_EncryptUpdate(3)` and `provider-cipher(7)`
to make the front-end and back-end documentation consistent.

Reviewed-by: Tomas Mraz <tomas at openssl.org>
Reviewed-by: Tom Cosgrove <tom.cosgrove at arm.com>
(Merged from https://github.com/openssl/openssl/pull/22875)

(cherry picked from commit 6ebdbba76a45294e22006ede1442847cdee24f03)