Clarify the EVP_DigestSignInit docs

They did not make it clear how the memory management works for the |pctx|
parameter.

Fixes #7037
Reviewed-by: NTim Hudson <tjh@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/7042)

doc/man3/EVP_DigestSignInit.pod

@@ -24,10 +24,12 @@ The EVP signature routines are a high level interface to digital signatures.
 
 EVP_DigestSignInit() sets up signing context B<ctx> to use digest B<type> from
 ENGINE B<e> and private key B<pkey>. B<ctx> must be created with
-EVP_MD_CTX_new() before calling this function. If B<pctx> is not NULL the
+EVP_MD_CTX_new() before calling this function. If B<pctx> is not NULL, the
 EVP_PKEY_CTX of the signing operation will be written to B<*pctx>: this can
-be used to set alternative signing options. The digest B<type> may be NULL if
-the signing algorithm supports it.
+be used to set alternative signing options. Note that any existing value in
+B<*pctx> is overwritten. The EVP_PKEY_CTX value returned must not be freed
+directly by the application (it will be freed automatically when the EVP_MD_CTX
+is freed). The digest B<type> may be NULL if the signing algorithm supports it.
 
 Only EVP_PKEY types that support signing can be used with these functions. This
 includes MAC algorithms where the MAC generation is considered as a form of

doc/man3/EVP_DigestVerifyInit.pod

@@ -23,9 +23,12 @@ The EVP signature routines are a high level interface to digital signatures.
 
 EVP_DigestVerifyInit() sets up verification context B<ctx> to use digest
 B<type> from ENGINE B<e> and public key B<pkey>. B<ctx> must be created
-with EVP_MD_CTX_new() before calling this function. If B<pctx> is not NULL the
+with EVP_MD_CTX_new() before calling this function. If B<pctx> is not NULL, the
 EVP_PKEY_CTX of the verification operation will be written to B<*pctx>: this
-can be used to set alternative verification options.
+can be used to set alternative verification options. Note that any existing
+value in B<*pctx> is overwritten. The EVP_PKEY_CTX value returned must not be
+freed directly by the application (it will be freed automatically when the
+EVP_MD_CTX is freed).
 
 EVP_DigestVerifyUpdate() hashes B<cnt> bytes of data at B<d> into the
 verification context B<ctx>. This function can be called several times on the