Update descriptions of build flags and the build guide

[harendra-kumar]

All the build flags in streamly (except maybe the has-llvm flag) are development only flags. We can make the flag descriptions more explicit mentioning that end-users should not try those unless they know what they are doing. The flags are unsupported for production use and are only to be enabled if recommended by the developers for a specific case (e.g. debugging something). We can mention this in the build guide as well.

An alternative is to remove the flags before uploading the release to hackage, but it has some downsides:

• Some flags like debug may be risky to use, but they could be useful for users to debug the code.
• Some flags like has-llvm even though not well-tested are likely to work and users who want to try them can still use.
• It will add additional overhead in the release process.

We should only remove the flags that change the API (dev and use-unliftio as of now).