common : use two decimal places for float arg help messages (#19048)

common : use two decimal places for float arg help messages (#19048)

* common : use two decimal places for float arg help messages

This commit updates the help messages for various command-line arguments
in arg.cpp to display floating-point default values with two decimal
places instead of one.

The motivation for this changes is that currently only having one decimal
place means that values generated using --help or llama-gen-docs will not
display the correct values.

For example, currently the value of top-p in tools/server/README.md is
`0.9`, but the default value is actually '0.95'. And running
llama-gen-docs does not update this value as it uses the output from the
help message, which shows only one decimal place, so the values look
like they are unchanged.

* docs : run llama-gen-docs to update docs