Document errors and warnings better (#6426)

Co-authored-by: Richard Hallows <jeddy3@users.noreply.github.com>

docs/user-guide/errors.md

@@ -4,26 +4,97 @@ In addition to rule problems, Stylelint surfaces the following errors and warnin
 
 ## CSS syntax error
 
-The chosen [PostCSS syntax](usage/options.md#customsyntax) was unable to parse the source.
+The CSS parser built into Stylelint, or the chosen [custom syntax](usage/options.md#customsyntax), could not parse your code.
+
+### Unclosed block
+
+You should balance your _curly braces_, typically of _declaration blocks_.
+
+For example:
+
+```diff css
+- a { color: red;
++ a { color: red; }
+```
+
+### Unclosed bracket
+
+You should balance your _square brackets_, typically of _attribute selectors_ and _grid names_.
+
+For example:
+
+```diff css
+- a[foo { grid: [bar; }
++ a[foo] { grid: [bar]; }
+```
+
+### Unclosed string
+
+You should balance your _quotation marks_.
+
+For example:
+
+```diff css
+- a { content: " }
++ a { content: "" }
+```
+
+### Unknown word
+
+You should:
+
+- hyphenate _properties_
+- separate _property and value pairs_ with _colons_
+- separate _declarations_ with _semicolons_
+- pair _selectors_ with _declaration blocks_ in _rules_
+
+For example:
+
+```diff css
+a {
+- margin top: 1px
+- color red
++ margin-top: 1px;
++ color: red
+}
+```
 
 ## Parse error
 
-The chosen [PostCSS syntax](usage/options.md#customsyntax) successfully parsed, but one of the construct-specific parsers failed to parse either a media query, selector or value within that source.
+The CSS parser built into Stylelint (or the chosen [custom syntax](usage/options.md#customsyntax)) successfully parsed your code, but one of the construct-specific parsers failed to parse either a media query, selector or value within that source.
 
 The construct-specific parsers are:
 
 - `postcss-media-query-parser`
 - `postcss-selector-parser`
 - `postcss-value-parser`
 
+You should check that your constructs are wellformed, e.g. parentheses are balanced.
+
 ## Unknown rule error
 
 There is an unknown rule in the [configuration object](configure.md).
 
+You should:
+
+- install the latest version of Stylelint (`npm i --save-dev stylelint`), as we may have recently added the rule
+- check that the rule exists and is correctly named by searching the [list of rules](rules.md)
+
 ## Deprecation warning
 
 There is a deprecated rule in the [configuration object](configure.md).
 
+You should:
+
+- identify in the [CHANGELOG](../../CHANGELOG.md) which release deprecated the rule
+- take the action suggested there
+
 ## Invalid option warning
 
 There is a misconfigured rule in the [configuration object](configure.md).
+
+You should:
+
+- install the latest version of Stylelint (`npm i --save-dev stylelint`), as we may have recently added the option
+- check that the option exists and is correctly named by reading the rule's README
+- correctly configure the [`rules`](configure.md#rules) property in the configuration object