build: reduce noise from doc target

The doc target currently echos the complete shell command, which might
produces a lot of output. Using this change the actual commands run
are still shown.

Before:
[ -e tools/doc/node_modules/js-yaml/package.json ] || \
        [ -e tools/eslint/node_modules/js-yaml/package.json ] || \
        if [ -x ./node ]; then \
            cd tools/doc && ../.././node ../.././deps/npm/bin/npm-cli.js
install; \
        else \
            cd tools/doc && node ../.././deps/npm/bin/npm-cli.js
install; \
        fi
[ -x ./node ] && ./node tools/doc/generate.js --node-version=v8.0.0
--format=html --template=doc/template.html doc/api/assert.md >
out/doc/api/assert.html || node tools/doc/generate.js
--node-version=v8.0.0 --format=html --template=doc/template.html
doc/api/assert.md > out/doc/api/assert.html
Input file = doc/api/assert.md

After:
[ -x ./node ] && ./node tools/doc/generate.js --node-version=v8.0.0
--format=html --template=doc/template.html doc/api/assert.md >
out/doc/api/assert.html || node tools/doc/generate.js
--node-version=v8.0.0 --format=html --template=doc/template.html
doc/api/assert.md > out/doc/api/assert.html
Input file = doc/api/assert.md

To verify run:
$ make docclean
$ make doc

PR-URL: https://github.com/nodejs/node/pull/9457


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: James Snell <jasnell@gmail.com>