docs: document array parameter support in std.stripChars/lstripChars/rstripChars

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Summary

• Document that std.stripChars, std.lstripChars, and std.rstripChars accept an array of single-character strings as the chars parameter
• Add examples showing array usage for each function

Motivation

The reference implementation in stdlib/std.jsonnet inherently supports arrays because it uses std.member(chars, str[0]), which works with both strings and arrays. This behavior is consistent across all major implementations (cpp-jsonnet, go-jsonnet, sjsonnet, jrsonnet) but is not documented.

When chars is an array:

• Single-character string elements are used as characters to strip
• Non-string elements and multi-character strings are silently ignored

Modification

Added a paragraph and example to each of the three function documentation sections in doc/ref/stdlib.html:

• std.stripChars
• std.lstripChars
• std.rstripChars

Result

Users can now discover and rely on the array parameter behavior, which is already supported by all implementations.

Cross-implementation verification

Expression	cpp-jsonnet	go-jsonnet	sjsonnet	jrsonnet
std.stripChars("abcba", ["a", "c"])	"bcb"	"bcb"	"bcb"	"bcb"
std.lstripChars("abcba", ["a", "c"])	"bcba"	"bcba"	"bcba"	"bcba"
std.rstripChars("abcba", ["a", "c"])	"abcb"	"abcb"	"abcb"	"abcb"

Closes #1325

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