chore(docs): reword CSS in JS docs for clarity (gatsbyjs#11439)

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## Description

Gives clear info about installation of libraries, helpers and gatsby plugins. Adds review comments from emotion docs.

## Related Issues

<!--
  Link to the issue that is fixed by this PR (if there is one)
  e.g. Fixes gatsbyjs#1234, Addresses gatsbyjs#1234, Related to gatsbyjs#1234, etc.
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yarn.lock

@@ -7298,17 +7298,17 @@ node-libs-browser@^2.0.0:
     vm-browserify "0.0.4"
 
 node-pre-gyp@^0.10.0:
-  version "0.10.0"
-  resolved "https://registry.yarnpkg.com/node-pre-gyp/-/node-pre-gyp-0.10.0.tgz#6e4ef5bb5c5203c6552448828c852c40111aac46"
-  integrity sha512-G7kEonQLRbcA/mOoFoxvlMrw6Q6dPf92+t/l0DFSMuSlDoWaI9JWIyPwK0jyE1bph//CUEL65/Fz1m2vJbmjQQ==
+  version "0.10.3"
+  resolved "https://registry.yarnpkg.com/node-pre-gyp/-/node-pre-gyp-0.10.3.tgz#3070040716afdc778747b61b6887bf78880b80fc"
+  integrity sha512-d1xFs+C/IPS8Id0qPTZ4bUT8wWryfR/OzzAFxweG+uLN85oPzyo2Iw6bVlLQ/JOdgNonXLCoRyqDzDWq4iw72A==
   dependencies:
     detect-libc "^1.0.2"
     mkdirp "^0.5.1"
-    needle "^2.2.0"
+    needle "^2.2.1"
     nopt "^4.0.1"
     npm-packlist "^1.1.6"
     npmlog "^4.0.2"
-    rc "^1.1.7"
+    rc "^1.2.7"
     rimraf "^2.6.1"
     semver "^5.3.0"
     tar "^4"
@@ -7383,9 +7383,9 @@ normalize-url@^3.0.0:
   integrity sha512-U+JJi7duF1o+u2pynbp2zXDW2/PADgC30f0GsHZtRh+HOcXHnw137TrNlyxxRvWW5fjKd3bcLHPxofWuCjaeZg==
 
 npm-bundled@^1.0.1:
-  version "1.0.3"
-  resolved "https://registry.yarnpkg.com/npm-bundled/-/npm-bundled-1.0.3.tgz#7e71703d973af3370a9591bafe3a63aca0be2308"
-  integrity sha512-ByQ3oJ/5ETLyglU2+8dBObvhfWXX8dtPZDMePCahptliFX2iIuhyEszyFk401PZUNQH20vvdW5MLjJxkwU80Ow==
+  version "1.0.5"
+  resolved "https://registry.yarnpkg.com/npm-bundled/-/npm-bundled-1.0.5.tgz#3c1732b7ba936b3a10325aef616467c0ccbcc979"
+  integrity sha512-m/e6jgWu8/v5niCUKQi9qQl8QdeEduFA96xHDDzFGqly0OOjI7c+60KM/2sppfnUU9JJagf+zs+yGhqSOFj71g==
 
 npm-conf@^1.1.0:
   version "1.1.3"
@@ -7396,9 +7396,9 @@ npm-conf@^1.1.0:
     pify "^3.0.0"
 
 npm-packlist@^1.1.6:
-  version "1.1.10"
-  resolved "https://registry.yarnpkg.com/npm-packlist/-/npm-packlist-1.1.10.tgz#1039db9e985727e464df066f4cf0ab6ef85c398a"
-  integrity sha512-AQC0Dyhzn4EiYEfIUjCdMl0JJ61I2ER9ukf/sLxJUcZHfo+VyEfz2rMJgLZSS1v30OxPQe1cN0LZA1xbcaVfWA==
+  version "1.2.0"
+  resolved "https://registry.yarnpkg.com/npm-packlist/-/npm-packlist-1.2.0.tgz#55a60e793e272f00862c7089274439a4cc31fc7f"
+  integrity sha512-7Mni4Z8Xkx0/oegoqlcao/JpPCPEMtUvsmB0q7mgvlMinykJLSRTYuFqoQLYgGY8biuxIeiHO+QNJKbCfljewQ==
   dependencies:
     ignore-walk "^3.0.1"
     npm-bundled "^1.0.1"