Merge pull request cli#862 from cli/cli-syntax

Add command line syntax doc

docs/command-line-syntax.md

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+# How we document our command line syntax
+
+## Literal text
+
+Use plain text for parts of the command that cannot be changed
+
+_example:_
+`gh help`
+The argument help is required in this command
+
+## Placeholder values
+
+Use angled brackets to represent a value the user must replace. No other expressions can be contained within the angeled brackets.
+
+_example:_
+`gh pr view <issue-number>`
+Replace `<issue-number>` with an issue number
+
+## Optional arguments
+
+Place optional arguments in square brackets. Mutually exclusive arguments can be included inside square brackets if they are separated with vertical bars.
+
+_example:_
+`gh pr checkout [--web]`
+The argument `--web` is optional.
+
+`gh pr view [<number> | <url>]`
+The `<number>` and `<url>` arguments are optional.
+
+## Required mutually exclusive arguments
+
+Place required mutually exclusive arguments inside braces, separate arguments with vertical bars.
+
+_example:_
+`gh pr {view | create}`
+
+## Repeatable arguments
+
+Ellipsis represent arguments that can appear multiple times
+
+_example:_
+`gh pr close <pr-number>...`
+
+## Variable naming
+
+For multi-word variables use dash-case (all lower case with words separated by dashes)
+
+_example:_
+`gh pr checkout <issue-number>`
+
+## Additional examples
+
+_optional argument with placeholder:_
+`command sub-command [<arg>]`
+
+_required argument with mutually exclusive options:_
+`command sub-command {<path> | <string> | literal}`
+
+_optional argument with mutually exclusive options:_
+`command sub-command [<path> | <string>]`