Improved description accuracy of pfx option

[naoki-san]

The pfx option is eventually passed to tls.createSecureContext(options).
According to the recent node.js reference, not only String but also Buffer is accepted.
For this reason, I modified the document to fit.

In real situation, I tried to use a pfx file for client authentication
but I was a bit confused by the description that type of pfx is String.
Because, pfx (PKCS#12) file is a binary file.

The kind of change this PR does introduce

• a bug fix
• a new feature
• an update to the documentation
• a code change that improves performance
• other

Current behaviour

New behaviour

Other information (e.g. related issues)

node.js v.4.x reference: https://nodejs.org/docs/latest-v4.x/api/tls.html#tls_tls_createsecurecontext_details
node.js v.6.x reference: https://nodejs.org/docs/latest-v6.x/api/tls.html#tls_tls_createsecurecontext_options
node.js v.8.x reference: https://nodejs.org/docs/latest-v6.x/api/tls.html#tls_tls_createsecurecontext_options

[darrachequesne]

Great, thanks! 👍