Fix several backslash typos in Elisp strings
* gnus-art.el (gnus-button-handle-library):
* gnus-group.el (gnus-read-ephemeral-gmane-group-url):
* nntp.el (nntp-via-shell-prompt, nntp-telnet-shell-prompt):
* spam-report.el (spam-report-gmane-regex):
Fix typo by replacing ‘\’ with ‘\\’ in a string literal.
For example, to get the regular expression ‘\.’ use the string
literal "\\.", not "\." (which is equivalent to ".").
Doc fixes
* auth-source.el (auth-source-read-char-choice):
* nnmaildir.el (NOV example):
* registry.el (registry-search):
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