[bazel][docs] Update build documentation

[rnk]

lgtm

[MaskRay]

Thank you for the update. You can consider CCing #bazel_build for future Bazel related changes.

utils/bazel/README.md

 
 # Quick Start
 
 1. `git clone https://github.com/llvm/llvm-project.git; cd llvm-project` if
    you don't have a checkout yet.
 2. Install Bazel at the version indicated by [.bazelversion](./.bazelversion),
    following the official instructions, if you don't have it installed yet:
    https://docs.bazel.build/versions/main/install.html.
+   * You can also install and use
+     [bazelisk](https://github.com/bazelbuild/bazelisk) which automates
+     downloading the proper bazel version
 3. `cd utils/bazel`
-4. `bazel build --config=generic_clang @llvm-project//...` (if building on Unix
-   with Clang/LLD). `--config=generic_gcc` and `--config=msvc` are also
-   available.
-
+4. `bazel build --config=generic_clang @llvm-project//...`
+   * If you're using clang, it's expected that lld is also available
+   * If you're using MSVC or gcc, instead of `--config=generic_clang`, pass
+   `--config=generic_gcc` or `--config=msvc`
+   * To specify a specific local compiler to use, add the following bazel
+     flag: `--repo_env=CC=/usr/bin/clang`
+     * `--config=generic_clang`/`--config=generic_gcc` by default set
+       `--repo_env=CC=clang`/`--repo_env=CC=gcc`, using `clang`/`gcc` on the
+       `PATH`
 
 # Configuration
 
 The repository `.bazelrc` will import user-specific settings from a
 `user.bazelrc` file (in addition to the standard locations). Adding your typical
 config setting is recommended.
 
 ```.bazelrc

 
 ```.bazelrc
 build --sandbox_base=/dev/shm
 ```
 
 Bear in mind that this requires that your ramdisk is of sufficient size to hold
 any temporary files. Anecdotally, 1GB should be sufficient.
 
-To specify a specific local compiler to use, add the following bazel flag:
-
-```.bazelrc
-build --repo_env=CC=$PATH_TO_CC
-```
-
 # Coverage
 
 The LLVM, MLIR, and Clang subprojects have configurations for Linux (Clang and
 GCC), Mac (Clang and GCC), and Windows (MSVC). Configuration options that are
 platform-specific are selected for in defines. Many are also hardcoded to the
 values currently used by all supported configurations. If there is a
 configuration you'd like to use that isn't supported, please send a patch.