merge revision(s) ef525b01,dc9a13ab: [Backport #18775]

	Explicit handling of frozen strings in `IO::Buffer#for`. (#5892)

	---
	 io_buffer.c                 | 122 +++++++++++++++++++++++++++++++++++---------
	 test/ruby/test_io_buffer.rb |  36 +++++++------
	 2 files changed, 117 insertions(+), 41 deletions(-)

	Fix rdoc of IO::Buffer [ci skip]

	---
	 io_buffer.c | 15 +--------------
	 1 file changed, 1 insertion(+), 14 deletions(-)