RFR: 8340804: doc/building.md update Xcode instructions to note that full install is required [v2]

[George Adams]

> Since JDK17 there has been a dependency on metal in the macOS builds which is only available as part of the full Xcode IDE (rather than command-line tools). We currently recommend only installing the command-line tools in the building.md doc which is incorrect.
> 
> See https://github.com/openjdk/jdk/blob/e1c4d3039f6b5106ce3f65d50f607eacc2a8d168/make/autoconf/toolchain.m4#L681

George Adams has updated the pull request incrementally with one additional commit since the last revision:

  run pandoc

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/21154/files
  - new: https://git.openjdk.org/jdk/pull/21154/files/6a491bf2..084cf7b2

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=21154&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=21154&range=00-01

  Stats: 3 lines in 2 files changed: 1 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/21154.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21154/head:pull/21154

PR: https://git.openjdk.org/jdk/pull/21154