lex: add examples and add {Free,Net,Open}BSD pages (#11895)

* lex: add examples and add other BSD

* lex: fix OpenBSD man link

* lex: remove unsupported option from OpenBSD

* lex: apply suggestions from code review

Co-authored-by: Juri Dispan <juri.dispan@posteo.net>
Co-authored-by: K.B.Dharun Krishna <kbdharunkrishna@gmail.com>

* flex, lex: moving flex options to common/flex.md

* lex: mention `lex` alias to `flex`

* Update pages/linux/lex.md

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Co-authored-by: Juri Dispan <juri.dispan@posteo.net>
Co-authored-by: K.B.Dharun Krishna <kbdharunkrishna@gmail.com>

pages/common/lex.md

@@ -2,9 +2,10 @@
 
 > Lexical analyzer generator.
 > Given the specification for a lexical analyzer, generates C code implementing it.
+> NOTE: on most major OSes, this command is an alias for `flex`.
 > More information: <https://manned.org/lex.1>.
 
-- Generate an analyzer from a Lex file:
+- Generate an analyzer from a Lex file, storing it to the file `lex.yy.c`:
 
 `lex {{analyzer.l}}`