bootstrap: optimizations could tell what "true" means

[matthiaskrgr]

# Whether or not to optimize when compiling the compiler and standard library,
# and what level of optimization to use.
# WARNING: Building with optimize = false is NOT SUPPORTED. Due to bootstrapping,
# building without optimizations takes much longer than optimizing. Further, some platforms
# fail to build without this optimization (c.f. #65352).
# The valid options are:
# true - Enable optimizations.
# false - Disable optimizations.
# 0 - Disable optimizations.
# 1 - Basic optimizations.
# 2 - Some optimizations.
# 3 - All optimizations.
# "s" - Optimize for binary size.
# "z" - Optimize for binary size, but also turn off loop vectorization.
#optimize = true

It's not clear what the default true corresponds to. Basic? Some? All?
I think its 3 but the config.toml should say that explicitly.