[ruby/bigdecimal] Enhanced RDoc for BigDecimal (ruby/bigdecimal#209)

* Enhanced RDoc for BigDecimal

* Update ext/bigdecimal/bigdecimal.c

Remove the instance number of `Float::DIG`.

* Update ext/bigdecimal/bigdecimal.c

Add BigDecimal call-seq without ndigits.

* Update ext/bigdecimal/bigdecimal.c

Replace the word sum with value or result in the description of BigDecimal().

* Update ext/bigdecimal/bigdecimal.c

Remove the instance value of Float::DIG.

* Update ext/bigdecimal/bigdecimal.c

Fix mis-description of precision

* Update ext/bigdecimal/bigdecimal.c

Fix the description of precision determination

* Update ext/bigdecimal/bigdecimal.c

Add the description of the precision in the Rational case.

ruby/bigdecimal@acabb132a4

Co-authored-by: Kenta Murata <3959+mrkn@users.noreply.github.com>