Document `&&` eager parsing in `python.gram`

[sobolevn]

Right now it is not documented:

cpython/Grammar/python.gram

Lines 45 to 81 in 5a4d3df

	# Grammar Syntax (see PEP 617 for more information):
	#
	# rule_name: expression
	# Optionally, a type can be included right after the rule name, which
	# specifies the return type of the C or Python function corresponding to the
	# rule:
	# rule_name[return_type]: expression
	# If the return type is omitted, then a void * is returned in C and an Any in
	# Python.
	# e1 e2
	# Match e1, then match e2.
	# e1 | e2
	# Match e1 or e2.
	# The first alternative can also appear on the line after the rule name for
	# formatting purposes. In that case, a | must be used before the first
	# alternative, like so:
	# rule_name[return_type]:
	# | first_alt
	# | second_alt
	# ( e )
	# Match e (allows also to use other operators in the group like '(e)*')
	# [ e ] or e?
	# Optionally match e.
	# e*
	# Match zero or more occurrences of e.
	# e+
	# Match one or more occurrences of e.
	# s.e+
	# Match one or more occurrences of e, separated by s. The generated parse tree
	# does not include the separator. This is otherwise identical to (e (s e)*).
	# &e
	# Succeed if e can be parsed, without consuming any input.
	# !e
	# Fail if e can be parsed, without consuming any input.
	# ~
	# Commit to the current alternative, even if it fails to parse.
	#

Linked PRs

• gh-118323: Document && grammar syntax #118324