doc: add documentation for request.path

[gjbkz]

The field has been added in v0.4.0.

Refs: b09c588#diff-1c0f1c434b17b7f8795d44a51a14320aR814
Refs: /test/parallel/test-http-client-defaults.js

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• documentation is changed or added
• commit message follows commit guidelines

I ran make -j4 test on macOS. But I got the message below:

=== release test-fs-stat-bigint ===                                           
Path: parallel/test-fs-stat-bigint
(node:95776) ExperimentalWarning: The fs.promises API is experimental
assert.js:351
    throw err;
    ^

AssertionError [ERR_ASSERTION]: atimeMs is not a safe integer, difference should < 1.
Number version 1548759595527.3901, BigInt version 1548759595525n
    at verifyStats (/Users/itok/node/test/parallel/test-fs-stat-bigint.js:72:7)
    at fs.stat (/Users/itok/node/test/parallel/test-fs-stat-bigint.js:109:7)
    at FSReqCallback.oncomplete (fs.js:160:5)
Command: out/Release/node /Users/itok/node/test/parallel/test-fs-stat-bigint.js
[04:35|% 100|+ 2518|-   1]: Done
make[1]: *** [jstest] Error 1
make: *** [test] Error 2

[gjbkz]

I added the commit and edited the description as I noticed that v0.3.6 didn't expose request.path.

[vsemozhetbyt]

CI-lite: https://ci.nodejs.org/job/node-test-pull-request-lite-pipeline/2475/

[vsemozhetbyt]

Landed in 32c7ad6
Thank you!